cmd/go: split out cmd/go/internal/str

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: I63f578f5ac99c707b599ac5659293c46b275567d
Reviewed-on: https://go-review.googlesource.com/36190


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