Fix for importing modules from definitions

Please see #492 for bug description.

Removed assertion in `getSourceFile` since `processSourceFile` obviously expects `findSourceFile` to return falsy value from `findSourceFile` (which is wrapper around `getSourceFile` result) when file is not found, and not to fail in that case.
I believe `CompilerHost`s interact with the compiler itself, where we entirely avoid the use of `null`. This should be `undefined`.