[ty] Add flow diagram for import resolution

The diagram is written in the Dot language, which can
be converted to SVG (or any other image) by GraphViz.

I thought it was a good idea to write this down in
preparation for adding routines that list modules.
Code reuse is likely to be difficult and I wanted to
be sure I understood how it worked.

crates/ty_python_semantic/src/module_resolver/resolver.rs

@@ -1,3 +1,13 @@
+/*!
+This module principally provides two routines for resolving a particular module
+name to a `Module`: [`resolve_module`] and [`resolve_real_module`]. You'll
+usually want the former, unless you're certain you want to forbid stubs, in
+which case, use the latter.
+
+For implementors, see `import-resolution-diagram.svg` for a flow diagram that
+specifies ty's implementation of Python's import resolution algorithm.
+*/
+
 use std::borrow::Cow;
 use std::fmt;
 use std::iter::FusedIterator;