cmd/go: fix module loader and test-only dependencies

go list all was not behaving as documented - it did not pick up
test dependencies except when running in "go test" and "go vet".
It should pick them up always.

Also the module loader was ignoring tests when using "go list -test",
which led to load failures.

Fixing all required adjustments to mod_patterns test.
Removed error-prone exact listings.

Fixes #26279.
Fixes #26906.

Change-Id: I9c5acaf2275be20fd2349859589502190d3e7a78
Reviewed-on: https://go-review.googlesource.com/128358


Reviewed-by: Bryan C. Mills <bcmills@google.com>