patch: split up fetch and clean into separate methods (#10150)

"mirror create" was invoking a package's do_patch method in order to
retrieve and archive URL patches. If a package implements a "patch"
method, this is also called as part of do_patch; this failed when the
package-specific implementation referred to environment variables
that are only available at the time the package is built
(e.g. "spack_cc").

This change introduces fetch and clean methods for patches. They are
no-ops for FilePatch but perform the appropriate actions for
UrlPatch. This allows "mirror create" to invoke do_fetch, which does
not call the package's patch method.

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