Search the local package folder only once

Previously, even with an SSD, a `New-Release` would take about 20 seconds for my project.

I changed the code, so that the local package folder is only searched once, and packages which dependencies are already looked up aren't looked up twice.

This makes the time for a `New-Release`go down to ~8 seconds for me.

EDIT: I've also created a method `GetAllFilePathsRecursively` that returns file paths, most of the time there is no need to return a `FileInfo` object.