Fall back to prebuilts if source modules are disabled

In lieu of having an environment variable to override prebuilts
preferences (for now), fall back to prebuilts whenever the source
module is disabled.  Complicated source modules (like LLVM) that
already have Go logic can then use an environment variariable to
disable their source module whenever they want to use prebuilts.

Computes UsePrebuilt once using information from both the source
and prebuilt modules, and stores the result in the prebuilt module.

Test: m -j libLLVM
Test: m -j libLLVM FORCE_BUILD_LLVM_COMPONENTS=true
Change-Id: Ib0819a03e9492e55f55de3402e05b3a043644a39