Swift: implement python schema

The information that was contained in `schema.yml` is now in
`swift/schema.py`, which allows a more integrated IDE experience
for writing and navigating it.

Another minor change is that `schema.Class` now has a `str` `group`
field instead of a `pathlib.Path` `dir` one.

swift/prefix.dbscheme

@@ -0,0 +1,6 @@
+/**
+ * The source location of the snapshot.
+ */
+sourceLocationPrefix(
+  string prefix: string ref
+);