Now that a URL, version, and the (for lack of a better term) "specs"
associated with said URL (e.g. the VCS revision, or a download strategy
hint) are neatly bundled up in a SoftwareSpec object, it doesn't make
sense to pass them individually to download strategy constructors. These
constructors now take only the formula name and a SoftwareSpec as
parameters.
This allows us to move mirror handling out out of Formula#fetch and into
the download strategies themselves. While doing so, we adjust the mirror
implementation a bit; mirrors now assume the same "specs" as their
owner's URL. They are still only useable by the CurlDownloadStrategy,
but this provides a basis for extending mirror support to other
strategies.
Signed-off-by: Jack Nagel <jacknagel@gmail.com>