Simplify resolution of include/extend/types/inline/extension files

[evhan]

There are currently a bunch of different implementations of "file resolution", many of which amount to the same thing and some of which seem to be (mis|ab)used in certain places. It would be good to audit these and simplify things where possible.

At a glance, load-identifier-database and load-types-file both have their own logic for locating files; ##sys#find-extension is similar to these in that it searches the working directory and the repository, but also considers "setup-mode" (speaking of which, is this still necessary with the new egg system?) and the include path when looking for import libraries; and then there's ##sys#resolve-include-filename, which is being used for include, extend, and inline files. This one is pretty complex and has a somewhat confusing signature. It would also be worth investigating whether ##sys#canonicalize-extension-path is really necessary wherever it appears.