dispatch: make loading extra information from extension extensible
This patch makes loading extra information from extension module at
dispatching extensible. Factoring 'loadcmdtable()' into commands.py is
a part of generalization of loading extra information.
This extensibility assumes registration of new function like below,
for example:
- revset predicate
- fileset predicate
- template keyword
- template filter
- template function
- internal merge tool
- web command
This patch requires not loader function itself but container module
and the name of it, because listing loader function directly up
implies actual loading module of it, even if it isn't used at runtime
(for example, extensions don't always define revset predicate)