cmd/go: split out cmd/go/internal/buildid

cmd/go: split out cmd/go/internal/buildid

This is one CL in a long sequence of changes to break up the
go command from one package into a plausible group of packages.

This sequence is concerned only with moving code, not changing
or cleaning up code. There will still be more cleanup after this sequence.

The entire sequence will be submitted together: it is not a goal
for the tree to build at every step.

For #18653.

Change-Id: I20dbc352c3df3c83a75811dd8e78c580a46b2202
Reviewed-on: https://go-review.googlesource.com/36195
Reviewed-by: David Crawshaw <crawshaw@golang.org>