debugcommands: move 'debugextensions' to the new module

debugcommands: move 'debugdiscovery' in the module And a lot of imports with it.

debugcommands: move 'debugdate' in the new module

debugcommands: move 'debugrevlogopts' into the new module This move contains the first reference to debugrevlogopts in debugcommands.py. We'll eventually want to move that over. We hold off for now because it would introduce a module import cycle.

debugcommands: move 'debugdag' into the new module

chgserver: make it a core module and drop extension flags It was an extension just because there were several dependency cycles I needed to address. I don't add 'chgserver' to extensions._builtin since chgserver is considered an internal extension so nobody should enable it by their config.

chgserver: delay importing commands and dispatch modules This is a workaround for future import cycle: dispatch -> commands -> server -> chgserver -> commands. Some of the problems can be fixed later on pager and chg refactoring.

chgserver: drop CHGINTERNALMARK by chgunixservice() Prepares for the removal of uisetup(). We just need to do that at the start of the chg server, so chgunixservice() should be fine.

server: add public function to select either cmdserver or hgweb

server: move service factory from hgweb

hgweb: extract app factory I'll move createservice() to the server module, but createapp() seems good to remain in the hgweb module because of its dependency on hgweb/hgwebdir_mod.

server: move service table and factory from commandserver This is necessary to solve future dependency cycle between commandserver.py and chgserver.py. 'cmd' prefix is added to table and function names to avoid conflicts with hgweb.