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.

% file replaced with directory
adding a
% should fail - would corrupt dirstate
abort: file 'a' in dirstate clashes with 'a/a'
% directory replaced with file
adding a/a
% should fail - would corrupt dirstate
abort: directory 'a' already in dirstate
% directory replaced with file
adding b/c/d
% should fail - would corrupt dirstate
abort: directory 'b' already in dirstate