go/internal/srcimporter: simplify and fix package file lookup

The old code was a blend of (copied) code that existed before go/build,
and incorrect adjustments made when go/build was introduced. This change
leaves package path determination entirely to go/build and in the process
fixes issues with relative import paths.

Fixes #23092
Fixes #24392

Change-Id: I9e900538b365398751bace56964495c5440ac4ae
Reviewed-on: https://go-review.googlesource.com/83415


Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>