This change moves from marking a package with `go list` errors as
missing, to marking a package with no files as missing.
Change-Id: Ibad1e67518d8a7f4c4bde416c53ab8132ae534e3
Reviewed-on: https://go-review.googlesource.com/c/tools/+/184039
Reviewed-by: Ian Cottrell <iancottrell@google.com>