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.