cmd/go/internal/modload: set errors for packages with invalid import paths

Prior to CL 339170, relative errors in module mode resulted in a
base.Fatalf from the module loader, which caused unrecoverable errors
from 'go list -e' but successfully rejected relative imports (which
were never intended to work in module mode in the first place).

After that CL, the base.Fatalf is no longer present, but some errors
that had triggered that base.Fatalf were no longer diagnosed at all:
the module loader left them for the package loader to report, and the
package loader assumed that the module loader would report them.

Since the module loader already knows that the paths are invalid,
it now reports those errors itself.

Fixes #51125

Change-Id: I70e5818cfcfeea0ac70e17274427b08a74fd7c13
Reviewed-on: https://go-review.googlesource.com/c/go/+/386176


Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>