cmd/compile: use correct packages when exporting/importing _ (blank) names
1) Blank parameters cannot be accessed so the package doesn't matter.
Do not export it, and consistently use localpkg when importing a
blank parameter.
2) More accurately replicate fmt.go and parser.go logic when importing
a blank struct field. Blank struct fields get exported without
package qualification.
(This is actually incorrect, even with the old textual export format,
but we will fix that in a separate change. See also issue 15514.)
Fixes #15491.
Change-Id: I7978e8de163eb9965964942aee27f13bf94a7c3c
Reviewed-on: https://go-review.googlesource.com/22714 Reviewed-by: Matthew Dempsky <mdempsky@google.com>