v23/generate: Fix external dependencies modules code in testmain.
v23/generate: Fix external dependencies modules code in testmain.

If only internal TestMain is created but external package needs modules,
ensure that internal TestMain generates the code.

Change-Id: I1b58f762bd82935810183d172ad51eeb4c3cf84a

vanadium/go.jiri,vanadium/go.jiri