Reland "Support per-module MakeVars"

This allows setting per-module make variables earlier in the build,
particularly for prebuilt_build_tool users like LEX/M4/BISON (which will
come in a later CL). I moved filegroup over because it's a simpler
common interface, but it doesn't strictly need it.

Test: Inspect out/soong/make_vars-*.mk, out/soong/late-*.mk
Test: treehugger
Change-Id: I38631129d82b0aad2438e83610bb6a97568bf932