cmd/go: split out cmd/go/internal/fmt

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: I8e325d75f553b5d0b6224b56a705d2e2cb895de4
Reviewed-on: https://go-review.googlesource.com/36199


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