Run app with @stlite/mountable without explicit file content embeddings

[whitphx]

Only with code like this, stlite should load app.py and run it.

stlite.mount({
  entrypointUrl: "./app.py",
  requirements: [...]
});

Even if the app.py imports other module e.g. from .foo import bar, stlite hooks the import and tries to load foo.py via HTTP access.

This makes it possible to serve an existing Streamlit project as a stlite app just by adding a single templated HTML file.