cmd/go: split out cmd/go/internal/env

cmd/go: split out cmd/go/internal/env

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: I28b20d53d20dff06eede574eb5c20359db0d3991
Reviewed-on: https://go-review.googlesource.com/36200
Reviewed-by: David Crawshaw <crawshaw@golang.org>