Correctly compute externalDep for dependencies of override_apex modules

The externalDep computation in WalkPayloadDeps was comparing
ctx.ModuleName() against InApexVariants, but override_apex modules
create an extra variation of the base apex, so ctx.ModuleName() doesn't
match the apex variation name.  Use the override name instead of the
module name for override apex variations.

Bug: 372543712
Test: go test ./apex
Change-Id: If7a92ca0034bb37f6b0fc7b194ea55c918469d08