cmd/go: split out cmd/go/internal/clean,doc,fix,generate,list,run,tool,version,vet

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: Ib22fc435827d4a05a77a5200ac437ce00e2a4da3
Reviewed-on: https://go-review.googlesource.com/36204


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