Detect import dependencies during setup phase

[janmasrovira]

The first step towards #2749, is to build the import tree during the setup phase. The strategy is to create a fast parser that scans all import statements in a Juvix file. In the pipeline, when we've added all dependencies, we'll use this parser to create the import tree. The import tree should have the paths already resolved (i.e. for each import statement, we know exactly to what other file in the system in points to).