cmd/compile: apply -importmap to imports before checking for package unsafe

cmd/compile: apply -importmap to imports before checking for package unsafe

There are fewer special cases this way: the import map applies
to all import paths, not just the ones not spelled "unsafe".

This is also consistent with what the code in cmd/go and go/build expects.
They make no exception for "unsafe".

For #13703.

Change-Id: I622295261ca35a6c1e83e8508d363bddbddb6c0a
Reviewed-on: https://go-review.googlesource.com/18438
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>