(stable) Allow bin/storage renamespace to work with underscores

If the namespace is something like "test_example" we currently fail to
renamespace the dump.

(Cowboy committing this since this is currently blocking a data export.)

Test Plan:

• Renamespaced a local dump, examined the output, saw 60 create / 60 use, reimported it.
• Will export in production.

Auditors: chad