cmd/go: split out cmd/go/internal/cfg

cmd/go: split out cmd/go/internal/cfg

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: Icb3f168ade91e7da5fcab89ac75b768daefff359
Reviewed-on: https://go-review.googlesource.com/36191
Reviewed-by: David Crawshaw <crawshaw@golang.org>