When getting the download directory name for a package that we want to
put in develop mode, a trailing slash in the URL or the subdir leads to
getting an empty string. To fix the problem we remove the trailing slash
from them at the start of the `getDevelopDownloadDir` procedure.

The `getDevelopDownloadDir` procedure is moved in the `download.nim`
module and units tests for it are added.

Fix the `getDevelopDownloadDir` procedure tests on Windows by adding
normalization of the expected paths.