cmd/go: split out cmd/go/internal/get

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: Iec17bf2243de129942ae5fba126ec5f217be7303
Reviewed-on: https://go-review.googlesource.com/36202


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