cmd/internal/str: move package from cmd/go/internal/str

This will let cmd/cgo and cmd/link use this package for argument parsing.

For golang/go#41400

Change-Id: I12ee21151bf3f00f3e8d427faaaab2453c823117
Reviewed-on: https://go-review.googlesource.com/c/go/+/334730


Trust: Jay Conrod <jayconrod@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-on: https://go-review.googlesource.com/c/go/+/341934


Run-TryBot: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>