Soong: Add BUILD_MODULES build action in soong_ui.

There was one case that did not work:
  1) Create dir in root source tree
  2) Run "m libc"

The build would fail as it was using the mma logic. A separate
one named BUILD_MODULES was added to allow building specific
modules in any directories using the "m" command.

Bug: b/130049705
Test: unit test cases, ran "m libc" in a tmp directory inside
      of the source tree.

Change-Id: I8d23e685a673a311001ee8edd89bd73b662392dd