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.
hg commands call extensions.loadall twice, once during dispatch and
once when the repository is instantiated. Without this change, load
caches successful loads, but not unsuccessful, causing errors to be
displayed twice.
