Reduce Startup Execution Time

[cyuria]

Reduces the impact of #1428

Nearly half of the startup time of running thefuck -a is caused by the removed line of code in thefuck/system/unix.py. This line of code imports find_executable from distutils, however this import takes on the order of 40% of the total startup time. This means my personal startup time has been reduced from about 400ms to 250ms or so as seen in the below image.

I've attempted to find further ways of reducing this time, but bar from fully isolating the entire chain of functions, it looks like this isn't particularly feasible.

One thing I haven't tried is to use lazy importing of some kind. This could yield serious performance improvements, as most of the delays are caused by import statements which, for the most part, aren't used by the --alias code path, however are not possible to easily move or remove due to their numerous quantity and interlinked dependencies in just about every source file. Transitioning to lazy loading would likely take a fair bit of effort to make sure everything still works and there is still no guarantee of better performance, however it is a possible avenue to continue down.

[cyuria]

It looks like this might conflict with #1404

I'd prefer that get merged over this PR, however I will leave this open until one of these two does get merged.