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.
% pattern error
grep: invalid match pattern: nothing to repeat!
% simple
port:4:export
port:4:vaportight
port:4:import/export
% all
port:4:4:-:spam:import/export
port:3:4:+:eggs:import/export
port:2:1:-:spam:import
port:2:2:-:spam:export
port:2:1:+:spam:export
port:2:2:+:spam:vaportight
port:2:3:+:spam:import/export
port:1:2:+:eggs:export
port:0:1:+:spam:import
% other
port:4:import/export
% follow
port:0:import
port2:6:4:+:eggs:deport
port:4:4:-:spam:import/export
port:3:4:+:eggs:import/export
port:2:1:-:spam:import
port:2:2:-:spam:export
port:2:1:+:spam:export
port:2:2:+:spam:vaportight
port:2:3:+:spam:import/export
port:1:2:+:eggs:export
port:0:1:+:spam:import
color:3:orange
color:3:+:orange
color:2:-:orange
color:1:+:orange
% issue 685
adding color
color:0:octarine
colour:1:octarine
colour:1:octarine