build: adjustments for move from src/pkg to src

This CL adjusts code referring to src/pkg to refer to src.

Immediately after submitting this CL, I will submit
a change doing 'hg mv src/pkg/* src'.
That change will be too large to review with Rietveld
but will contain only the 'hg mv'.

This CL will break the build.
The followup 'hg mv' will fix it.

For more about the move, see golang.org/s/go14nopkg.

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/134570043