cmd/internal/moddeps: confusing error message when difference is due to a "replace" directive

On CL 295150 Patchset 2, I suspected that the TryBots would fail due to the replace directive, but I wanted to see what would happen anyway.

TestDependencyVersionsConsistent did indeed fail as expected, but the last error in the output is very weird:

--- FAIL: TestDependencyVersionsConsistent (0.00s)
    moddeps_test.go:361: cannot check consistency for filesystem-local replacement in module cmd (/tmp/workdir/go/src/cmd):
        # golang.org/x/mod v0.4.1 => /usr/local/google/home/bcmills/x/mod
    moddeps_test.go:379: Modules within GOROOT require different versions of golang.org/x/mod.
    moddeps_test.go:391: cmd	requires v0.4.1 => /usr/local/google/home/bcmills/x/mod
    moddeps_test.go:391: cmd	requires =>

The correct reason for the failure is the one reported at line 361 (cannot check consistency for filesystem-local replacement).

However, the failure also includes an extraneous line cmd requires =>, which is quite confusing.