Add text about Pyodide package loading as well as the stdlib

Author: hoodmane

peps/pep-0776.rst

@@ -230,6 +230,9 @@ important of these are as follows:
 File System Setup
 -----------------
 
+The Standard Library
+====================
+
 In order for Python to run, it needs access to the standard library in the
 Emscripten file system. There are several possible approaches to this:
 
@@ -258,10 +261,35 @@ Emscripten file system. There are several possible approaches to this:
   in the browser. The disadvantage is that Emscripten does not itself include a
   TARFS so it requires a downstream implementation.
 
-Pyodide uses the ``ZipImporter`` approach in every runtime.
+Pyodide uses the ``ZipImporter`` approach in every runtime. Python uses the
+NODEFS approach when run with node and the ``ZipImporter`` approach for the web
+example. We will continue with this approach.
+
+The `ZipImporter` provides a clean resolution for a bootstrapping problem: The
+Python runtime is capable of unpacking a wide variety of archive formats, but
+the Python runtime is not ready to use until the standard library is already
+available. Since `zipimport.py` is a frozen module, it avoids these problems.
+All of the other approaches solve the bootstrapping problem by setting up the
+standard library using JavaScript.
+
+Packages
+========
 
-Python uses the NODEFS approach when run with node and the ``ZipImporter``
-approach for the web example. We will continue with this approach.
+It is also necessary to make any needed packages available in the Emscripten
+file system. Currently Emscripten cpython has no support for packages. Pyodide
+uses two different approaches for packages:
+
+* In the browser, Pyodide downloads and unpacks wheels into the MEMFS
+  site-packages directory. It then preloads all dynamic libraries in the wheel.
+  The work of downloading and installing all the packages is redone every time
+  the runtime starts.
+
+* The Pyodide `python` cli entrypoint mounts all of the host file system as
+  NODEFS directories before it bootstraps Python. This allows the normal virtual
+  environment mechanism to work. Pyodide virtual environments contain a patched
+  copy of pip and a custom `pip.conf` so that pip will install Pyodide wheels.
+  On startup the Pyodide `python` cli will preload all Emscripten dynamic
+  libraries that are in the sitee-packages directory.
 
 
 -----------------------------