Add a basic cangie.h header

Nothing here yet, it's coming...
Prevent including directly the internal headers

Allowing the inclusion of only the top-level cangjie.h header simplifies
the API and the guarantees we give on it.
Fix finding of the overrides install dir

The way we searched for it meant that it would hardcode the installation
prefix to the one from Python, not to what was specified as a configure
argument.

Turns out that we can make things much simpler, and they just work as
expected.