go/loader: name unnamed packages so errors make sense

Add tests for:
- this naming
- missing file in created package

Change-Id: I07c66b66e845c52d4685509c362b34f1f0c92648
Reviewed-on: https://go-review.googlesource.com/6310
Reviewed-by: Robert Griesemer <gri@golang.org>
2 files changed