cmd/go: split out cmd/go/internal/base

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: I7c5dde6e7fe4f390e6607303b4d42535c674eac3
Reviewed-on: https://go-review.googlesource.com/36193


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