Refactor pyexec (#1318)

This is some refactoring I did on the way towards resolving pyscript#1313.
I added a new _run_pyscript Python function which executes the code
inside a context manager that sets the display target. We can then
return a JS object wrapper directly from Python.

I moved the "installation" of the pyscript module to loadInterpreter,
and pyimport pyscript_py there and give it a type. This avoids a bunch
of creating and deleting of proxies for pyscript_py and allows us to
give it a type once and for all.

I also did some minor logic cleanup in a few places.

pyscriptjs/src/python_package.ts

@@ -0,0 +1,9 @@
+// This file exists because I can only convince jest to mock real file system
+// files, not fake modules. This confuses me because jest.mock has a virtual
+// option for mocking things that don't live in the file system but it doesn't
+// seem to work.
+
+// @ts-ignore
+import python_package from 'pyscript_python_package.esbuild_injected.json';
+declare const python_package: { dirs: string[]; files: [string, string][] };
+export { python_package };