cmd/go: split out cmd/go/internal/help

cmd/go: split out cmd/go/internal/help

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: I4cf05b076d81b780c87a31378523929b5da8964b
Reviewed-on: https://go-review.googlesource.com/36194
Reviewed-by: David Crawshaw <crawshaw@golang.org>