cmd/go: split out cmd/go/internal/bug

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: I05629567cc33fef41bc74eba4f7ff66e4851343c
Reviewed-on: https://go-review.googlesource.com/36203


Reviewed-by: David Crawshaw <crawshaw@golang.org>