cmd/go/internal/modload: report errors explicitly from Lookup
Previously, we reported errors directly in (*loader).load via base.Errorf.
Unfortunately, (*loader).load can be called from contexts in which such errors
should not be considered fatal, such as by load.PackagesAndErrors.
Instead, we save the errors in pkg.err and modify Lookup to return that error.
This change is a bit awkward: we end up suppressing a "no Go files" error for
packages at the root of newly-imported modules, even if they really do contain
source files. I believe that that's due to a special-case lookup for modules in
the build list, which allows us to "validate" imports for modules in the build
list even though we haven't actually downloaded their sources (or verified that
they actually contain the requested package). The fix for that issue is in the
change that follows this one.