By using a different overload of `retryDownload`,
we return the data instead of writing to disk,
ensuring that we don't have a lingering file
in memory and reducing IO.

This follow the same approach for deprecations as previous
`PackageSupplier` deprecations: it's an inevitable, but
easily fixable, breaking change for classes that implement
`PackageSupplier`, but it's a proper deprecation for client
code, based on the assumption that all the classes implementing
`PackageSupplier` are in Dub.

Since we recently did a semi-breaking change to PackageSuppliers,
now is the time to do it again (they will be in the same release).
Again the same logic applies: it's only a breaking change for those
inheriting from PackageSupplier, not those calling it,
and there is likely not a lot of people doing either out there.

This is a leftover of the time when everything in dub accepted Dependency.
While the mess had been untangled, doing so on this interface proved harder,
as there is no good way to deprecate it in a fully backward-compatible manner
without introducing a lot of overhead (a new type). Instead, this uses
a middle ground approach, where the external implementers (which most likely
do not exists) would break (although the fix is trivial), but callers would not.