[dev.inline] cmd/compile: split mkpackage into separate functions

Previously, mkpackage jumbled together three unrelated tasks: handling
package declarations, clearing imports from processing previous source
files, and assigning a default value to outfile.

Change-Id: I1e124335768aeabfd1a6d9cc2499fbb980d951cf
Reviewed-on: https://go-review.googlesource.com/35124


Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>