.mpdl format: Allow baseURL to end to '/'
Modpack installer forms the final URL by "baseURL + '/' + path". That means that baseURL is not allowed to contain the '/' as it would duplicate, leading to trouble as I just witnessed debugging problems on a modpack that was installed with baseURL "./"
We definitely be smarter about that, and check if baseURL already ends to '/', and not duplicate it in that case.
This should be the case both with paths relative to modpack list, and paths relative to .mpdl.
S3_0 (d3f branch) patch just adds warning about the format to README.modpack_installer.