Revert "Revert "Soong support for namespaces""

This mostly reverts commit 178d5fef
and mostly reapplies change I6d3e52ef62c4cabe85b9a135a54de0e1a6aab29c .

Bug: 65683273
Test: build/soong/scripts/diff_build_graphs.sh \
      --products=aosp_arm \
      'build/blueprint:work^ build/soong:work^' \
      'build/blueprint:work build/soong:work'
      # and see that the only changes were:
      # 1. adding some new files
      # 2. changing some line numbers
Test: m -j nothing # which runs unit tests

Change-Id: I32baae00277a547fdcdd1c2219fe6625ee0e45d7