Command completion types are now identified by ArgInfo, so just use
that directly and cut out the middle-man. This shouldn't change any
completion behavior.
Adds a test for get_pos_arg_info to test_cmdutils.
Modifies test_completer to test the use of get_pos_arg_info. Instead of
using FakeCommand, real Command objects are used, to validate that the
Completer works with the real Command interface. This also cleans out
some test cases that were testing things already covered by other cases.
c792ffda67