Move cmdtable and reposetup handling out of extensions.py
A new function (extensions.extensions) allows the code that is
interested in those attributes to handle them directly.
This allows some cleanups of extensions.py. Notably, we can
remove the extensions.commandtable hack.
It also makes it easier to add standard extension attributes,
like a "hgwebsetup" function or a "helptable" dict that augments
the data in help.py, etc.
os = <unloaded module 'os'>
os.system = <built-in function system>
os = <module 'os' from '?'>
mercurial.version = <unloaded module 'version'>
mercurial.version.get_version = <function get_version at 0x?>
mercurial.version = <module 'mercurial.version' from '?'>
mercurial = <module 'mercurial' from '?'>
util = <unloaded module 'util'>
util.system = <function system at 0x?>
util = <module 'mercurial.util' from '?'>
util.system = <function system at 0x?>
fred = <unloaded module 're'>
re = <unloaded module 'sys'>
fred = <unloaded module 're'>
fred.sub = <function sub at 0x?>
fred = <proxied module 're'>
re = <unloaded module 'sys'>
re.stdout = <open file '<stdout>', mode 'w' at 0x?>
re = <proxied module 'sys'>