Make override modules compatible with prebuilts.

Prebuilt's prefer flag doesn't work against override modules because
override modules are sort of virtual modules that delegate all the work
to their base modules. Therefore, even if a prebuilt module suppresses
installation of its src-counterpart override module, the actual build
actions are still performed in the base module.

This change fixes it by filtering out override modules that are being
replaced by prebuilts.

Test: prebuilt_test.go
Bug: 152155285
Change-Id: I859b35c0629b2b6258dd1ec5e020ba2c77ff9612