cmd/go: break a few dependencies

This CL makes a few naming changes to break dependencies
between different parts of the go command, to make it easier
to split into different packages.

This is the first 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: I69a98b9ea48e61b1e1cda95273d29860b525415f
Reviewed-on: https://go-review.googlesource.com/36129


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