checksymexport: rename, cleanup, and tuning

Now fast enough to be used as a post-commit git hook with no noticeable
performance hit.

* rename symexportcheck -> checksymexport
* remove external modules, just loading them was half the run time
* only search code files in the same directory as the header
* only take a single header at a time

Signed-off-by: Andrew Gregory <andrew.gregory.8@gmail.com>