bake: use shared session for local sources for multiple targets

[tonistiigi]

fixes #1377
fixes #92
depends on #2607

Detect cases where multiple bake targets would use the same
local source. For such cases a separate session request is
made in addition to session per target and local source
is made available in that source as well.

The new sessionID is sent with the request so the frontend
can ask associate it with the local source it needs.

The sources are still available in the main request session
as well. This would be used if frontend ignores the local-sessionid
parameter and makes sure that old version continue working.

> docker buildx bake lint validate-golangci

Before:

 => [validate-golangci internal] load build definition from lint.Dockerfile                                                        0.0s
 => => transferring dockerfile: 2.36kB                                                                                             0.0s
...
 => [validate-golangci internal] load build definition from lint.Dockerfile                                                        0.1s
 => => transferring dockerfile: 2.36kB                                                                                             0.0s
 => [lint internal] load build definition from lint.Dockerfile                                                                     0.1s
 => => transferring dockerfile: 2.36kB                                                                                             0.0s
 => [lint internal] load metadata for docker.io/library/golang:1.22-alpine                                                         0.2s
 => [validate-golangci internal] load metadata for docker.io/tonistiigi/xx:1.3.0                                                   0.3s
 => [validate-golangci internal] load .dockerignore                                                                                0.0s
 => => transferring context: 45B                                                                                                   0.0s
 => [lint internal] load .dockerignore                                                                                             0.1s
 => => transferring context: 45B                                                                                                   0.0s
...
 => [validate-golangci internal] load build context                                                                                0.6s
 => => transferring context: 960.30kB                                                                                              0.5s
...
 => [lint internal] load build context                                                                                             3.9s
 => => transferring context: 281.76MB
 
After:

  => [lint internal] load build definition from lint.Dockerfile                                                                     0.0s
  => => transferring dockerfile: 2.36kB                                                                                             0.0s
  => [lint internal] load metadata for docker.io/library/golang:1.22-alpine                                                         0.4s
  => [lint internal] load metadata for docker.io/tonistiigi/xx:1.3.0                                                                0.4s
  => [lint internal] load .dockerignore                                                                                             0.0s
  => => transferring context: 45B                                                                                                   0.0s
  => [validate-golangci golang-base 1/2] FROM docker.io/library/golang:1.22-alpine@sha256:8c9183f715b0b4eca05b8b3dbf59766aaedb41ec  0.0s
  => => resolve docker.io/library/golang:1.22-alpine@sha256:8c9183f715b0b4eca05b8b3dbf59766aaedb41ec07477b132ee2891ac0110a07        0.0s
  => [validate-golangci internal] load build context                                                                                0.5s
  => => transferring context: 957.98kB                                                                                              0.4s
  => [validate-golangci xx 1/1] FROM docker.io/tonistiigi/xx:1.3.0@sha256:904fe94f236d36d65aeb5a2462f88f2c537b8360475f6342e7599194  0.0s
  => => resolve docker.io/tonistiigi/xx:1.3.0@sha256:904fe94f236d36d65aeb5a2462f88f2c537b8360475f6342e7599194f291fb7e               0.0s
  => CACHED [validate-golangci golang-base 2/2] RUN apk add --no-cache git gcc musl-dev                                             0.0s
  => CACHED [validate-golangci lint-base 1/3] RUN wget -O- -nv https://raw.githubusercontent.com/golangci/golangci-lint/master/ins  0.0s
  => CACHED [validate-golangci lint-base 3/3] WORKDIR /go/src/github.com/docker/buildx                                              0.0s
  => CACHED [lint lint 1/1] RUN --mount=target=/go/src/github.com/docker/buildx     --mount=target=/root/.cache,type=cache,id=lint  0.0s
  => CACHED [lint lint-base 2/3] COPY --link --from=xx / /                                                                          0.0s
  => CACHED [validate-golangci validate-golangci 1/1] RUN --mount=target=/go/src/github.com/docker/buildx   golangci-lint config v  0.0s

This requires BuildKit/Dockerfile frontend with moby/buildkit#5174 support to work.

Note that this only works if the targets are using the same files from local context. If they ask for a different set of files then still multiple transfers will happen(on the same session). Atm. Dockerfile will try to pull all the files it needs from build context with a single transfer because every extra transfer results in new disk scanning overhead from the client side.

@dgageot