[PATCH 0 of 2] fixes hgweb's module loading phase (issue1824)

[Yuya Nishihara]

previously hgweb never calls extsetup().

this patch can fix the issue indirectly by moving responsibility
of handling of extsetup to extensions.loadall().

However this can affect third-party tools, e.g. TortoiseHg, so
I'll post another solution later.