cmd/compile: refactor import statement parsing

cmd/compile: refactor import statement parsing

Combine parser's import_stmt and import_here methods as a single new
importdcl method, and cleanup conditional logic slightly to make the
code easier to follow.

Also, eliminate importfile's unused line parameter, and get rid of all
of its duplicate type assertions.

Change-Id: Ic37ae8490afedc533f98ead9feef383e3599bc01
Reviewed-on: https://go-review.googlesource.com/19629
Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>