policy: add helper to import cext/pure module
These functions are sysstr API since __import__() and getattr() hate byte
strings on Python 3.
There's a minor BC, which is ImportError will be raised if invalid
HGMODULEPOLICY is specified. I think this is more desired behavior.
We're planning to add strict checking for C API compatibility. This patch
includes the stub for it.
http://example.com/no/anchor, branches: (None, [])
http://example.com/an/anchor, branches: ('foo', [])
http://example.com/no/anchor/branches, branches: (None, ['foo'])
http://example.com/an/anchor/branches, branches: ('bar', ['foo'])
http://example.com/an/anchor/branches-None, branches: ('foo', [])
http://example.com/, branches: (None, [])
http://example.com/, branches: (None, [])
http://example.com/, branches: ('foo', [])