Add --tag argument to soongdbg

The --tag argument will filter the dependencies to just the given deptags
as it walks the graph.

Test: soongdbg deps --tag android/soong/android.defaultsDependencyTag framework-minus-apex
Test: soongdbg between --tag android/soong/android.defaultsDependencyTag framework-minus-apex framework-minus-apex-defaults
Change-Id: I566477ffcd9d5e4b3e57155f65e84999e4908c79