cmd/go: include packages with InvalidGoFiles when filtering main packages

If a package has files with conflicting package names, go/build
empirically populates the first name encountered and puts the
remaining files in InvalidGoFiles. That foiled our check for packages
whose name is either unpopulated or "main", since the "package main"
could be found in a source file after the first.

Instead, we now treat any package with a nonzero set of InvalidGoFiles
as potentially a main package. This biases toward over-reporting
errors, but we would rather over-report than under-report.

If we fix #45999, we will be able to make these error checks more
precise.

Updates #42088
Fixes #45827
Fixes #39986

Change-Id: I588314341b17961b38660192c2130678dc03023e
Reviewed-on: https://go-review.googlesource.com/c/go/+/317300


Trust: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>