Ignore variable assignments that come after a rule definition

These private variables sometimes have references to make
variables like $< or $@. When converted to starlark, they
become invalid local variables like _< and _@, preventing
the file from being loaded even if it's never executed.

Bug: 226974242
Test: go test
Change-Id: Iafd4c6939731f3b7c051c9e41464134d5b672f23