cmd/go: split out cmd/go/internal/work

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: Icdd181098f9f0e81f68bf201e6867cdd8f820300
Reviewed-on: https://go-review.googlesource.com/36197


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