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.
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.