build(golang): Support faster local iteration and align better with u…

…pstream

* Replace synthetic "go-mod-cache" Makefile target with direct dependence
  upon go.sum.
* Introduce support for `make SKIP_MOD_VERIFY=1` to avoid unnecessary
  expensive scans when e.g. tweaking referents of local-directory
  `replace $modulePath => ../path/to/module` directives in go.mod.

golang/cosmos/Makefile

@@ -61,10 +61,10 @@ node-compile-gyp:
 		npm run build:gyp; \
 	fi
 
-compile-agd: go-mod-cache
+compile-agd: go.sum
 	go build -v $(MOD_READONLY) $(BUILD_FLAGS) -buildmode=exe -o build/agd ./cmd/agd
 
-install-agd: go-mod-cache
+install-agd: go.sum
 	go install -v $(MOD_READONLY) $(BUILD_FLAGS) -buildmode=exe ./cmd/agd
 
 # Only run from the package.json build:gyp script.
@@ -73,17 +73,18 @@ compile-gyp:
 	node-gyp configure build $(GYP_DEBUG) || { status=$$?; rm -f binding.gyp; exit $$status; }
 	rm -f binding.gyp
 
-compile-libdaemon: go-mod-cache
+compile-libdaemon: go.sum
 	go build -v $(MOD_READONLY) $(SHARED_BUILD_FLAGS) -buildmode=c-shared \
 		-o build/libagcosmosdaemon.so ./cmd/libdaemon/main.go
 
-go-mod-cache: go.sum
-	@echo "--> Download go modules to local cache"
-	@go mod download
-
 go.sum: go.mod
-	@echo "--> Ensure dependencies have not been modified"
+	@echo "--> Ensure dependencies have not been modified unless suppressed by SKIP_MOD_VERIFY ..."
+ifndef SKIP_MOD_VERIFY
 	GO111MODULE=on go mod verify
+endif
+	GO111MODULE=on go mod tidy
+	@echo "--> Download go modules to local cache"
+	go mod download
 
 ###############################################################################
 ###                           Protobuf                                    ###