[dev.regabi] cmd/compile: cleanup import/export code

Now that we have concrete AST node types and better constructor APIs,
we can more cleanup a lot of the import code and some export code too.

Passes toolstash -cmp.

Change-Id: Ie3425d9dac11ac4245e5da675dd298984a926df4
Reviewed-on: https://go-review.googlesource.com/c/go/+/279954


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>