Extract import code into a function and unit test it

[wendi-yu]

The code that currently produces the import_folder dict is inline and only able to look at the imports folder where we keep actual imports, which isn't very conducive to unit testing. We should pull it out into a function that takes the path to look for the folder in as an argument, and that also has a mode that yields a fake list of paths to test with. A function signature could look something like this:

def get_imports(folder_path, mockPaths=None)`

where folder_path is the path to a folder, and mockPaths is an optional list of fake paths to be used for unit testing. Depending on the value of mockPaths, the function either trawls the provided folder path for the list of files or uses the given list directly.

We can also ignore files without the .pdl extension in in this function, so that we have more flexibility in what goes into the imports folder.