cmd/dist: move cmd/go z files to cmd/go/internal/cfg

cmd/dist: move cmd/go z files to 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: I6ee5b053683034ea9462a9a0a4ea4f5ad24fa5a1
Reviewed-on: https://go-review.googlesource.com/36192
Reviewed-by: David Crawshaw <crawshaw@golang.org>