Don't change directories when interpreting files

[samcowger]

A while ago, #17 was originally fixed by changing directories when interpreting files. Recently, in #1770, a latent bug introduced by this behavior was fixed. As @kquick mentions here, though, this directory-changing behavior is a bit clumsy by nature.

The better thing to do would be to maintain the "current" directory in an environment, and when interacting with a file system, prepend that directory to the filename as it appears. This will still allow for the relative imports the author of #17 desired, while more neatly eliminating the bug fixed in #1770.

This is low-priority, especially if the likes of SAWScript.Interpreter.interpretFile is never used in a library context, but the fix would still benefit the health of the codebase.