moving the source dir scanning logic into a function (in preparation for
further refactoring of single-file or entire-directory source document
detection)
adapting tests (by changing the name from SourceCollection to scansourcedirs).
no other tests required
added new test to ensure that an empty SourceCollection() returned as expected
check to see that empty directories are handled correctly
check to see that single, multiple files in one dir are handled
check to see that single files in multiple directories are handled