Better Soong namespace support in the RBC runtime.

Includes:
* Defining namespace again should have no effect
* Allow appending to a variable defined in a namespace
* Consistent naming
* Print namespace variable assignments even if they are empty

Bug: 200297238
Test: rbcrun make/build/tests/run.rbc
Change-Id: I64aa22c4498ae89e4bc3a077d6206ad37d5c7c38