Make the subrepo targets output into plz-out/gen based on their pkg dir

[Tatskaari]

This was a bit weird. We have the package root set, but it had no bearing on where we stash this guy in plz-out. Before and after:

➜  please git:(subrepo-target-out-dir) ✗ plz build ///third_party/go/github.com_stretchr_testify//assert 
Build finished; total time 480ms, incrementality 80.0%. Outputs:
///third_party/go/github.com_stretchr_testify//assert:assert:
  plz-out/gen/third_party/go/github.com_stretchr_testify/assert/assert.a

➜  please git:(subrepo-target-out-dir) ✗ plz plz build ///third_party/go/github.com_stretchr_testify//assert
Build finished; total time 670ms, incrementality 44.2%. Outputs:
///third_party/go/github.com_stretchr_testify//assert:assert:
  plz-out/gen/pkg/linux_amd64/github.com/stretchr/testify/assert/assert.a

This has no real functional effect, but seems like it might be the right thing to do.

[peterebden]

Can you please check whether this has some impact on what the Go package driver should be doing with these paths? Feels like it might break it again (although I agree it's probably more sensible this way)