Fix problem with Program::parse()

[Radvendii]

Program::parse() was calling ImportResolver::get() which assumed that as an ImportResolver, we can only get terms when all the imports are resolved, so it had a debug_assert!() to ensure this. In this case, we are only parsing the file and returning its AST, so we can get the terms field of the Cache and call get() on that.

We had discussed a much more complicated solution that essentially amounted to this, plus a refactor of how ImportResolver works. But as I was implementing it started seeming less like a good idea, and also I realized not necessary to solve this problem. We can revisit that refactor, but for now this solves the problem.