sources: source itself controls cache sub-dir

Instead of supplying the full cache dir, i.e. the directory in
the store where the source will place the fetched resources, to
the source, only supply the root folder of the cache and let
the source itself create the desired sub-directory. This allows
the source to determine what type of resource it provides. This
makes the final directory independent of the name of the source:
a `org.osbuild.curl` source can place file-like resource in the
`org.osbuild.files` sub-directory. Then the `org.osbuild.files`
input can be used to get those from the cache directory.

osbuild/sources.py

@@ -20,7 +20,7 @@ class Source:
 
     def download(self, store: ObjectStore, libdir: PathLike):
         source = self.info.name
-        cache = os.path.join(store.store, "sources", source)
+        cache = os.path.join(store.store, "sources")
         msg = {
             "items": self.items,
             "options": self.options,
@@ -73,7 +73,7 @@ class SourcesServer(api.BaseAPI):
         msg = {
             "items": {},
             "options": self.options.get(source, {}),
-            "cache": f"{self.cache}/{source}",
+            "cache": self.cache,
             "output": f"{self.output}/{source}",
             "checksums": checksums,
             "libdir": self.libdir