[feature] source downloads and the cache

A few things
- I noticed that conan.tools.files.get() doesn't seem to use the download cache. So I reverted back to the conans get tool. I was trying to upgrade away from the old conans tools.

- I'd like to be able to tell conan to "download" from a folder on my disk, ie url: file:/// if necessary

- Do not use the internet at all if the source file is in the download-cache folder.
I had VTK-9.1.0.tar.gz already downloaded. When I do "conan create" it download and unpacks quickly.
When my internet was disconnected, conan was not able to execute the create command - hitting the internet for some reason?