Please v8.0.0

This is Please v8.0.0

• The tarball rule has been rewritten to be more deterministic. As a consequence it now
  only supports gzip compression; the previous compression argument has been removed.
  This is obviously a breaking change for anything that was setting that argument.
• Build rules with needs_transitive_deps = True no longer receive transitive deps of
  their srcs, only their deps.
  This is again a breaking change although a relatively subtle case that doesn't affect
  many rules; most cases that were using srcs and deps semantically correctly won't notice.
• plz query print now accepts a --fields argument to print only the given fields.
• plz run now has two new subcommands, sequential and parallel that allow passing
  multiple targets to be run either consecutively or concurrently.
• Go 1.8 is now required to build the Please repo. Support for targeted Go versions is
  unchanged.