Move from all-URL to Path/URL enum

Move from all-URL to Path/URL enum

On windows a path cannot be represented as a file:// URL because of the
backslashes and colons in the file name. This causes all of the tests which rely
on git to fail on windows. This commit changes the representation of the
location of a package to be an enum, Location, with two variants: Remote and
Local.

When parsing Cargo.toml, all locations which begin with the string "file:" have
that prefix stripped and are then interpreted as Local packages. Everything else
is parsed as a URL and used as a Remote package.