libgl is marked .PHONY and is a dependency for the link step which forces us to always redo the link
step even though it's completely not necessary. I moved embuilder build libgl into the link step
since that's the only place where it's needed.
dist/python_stdlib.zip and dist/pyodide-lock.json both declare a dependency on pyodide_build but
there is never a file of that name so they always have to run. Instead, run make pyodide_build so
that they can reuse the logic but they don't always run if their other deps are satisfied.
Now that dist/python_stdlib.zip doesn't always run it needs to rely on all files in src/py.
Make pyodide_build only invoke pip install if pyodide_build is not already installed. Pip generates
a lot of noise and takes a while so skipping it when we don't need it helps a lot.
Make dist/webworker.js and friends not phony since they stopped needing to be when PYODIDE_BASE_URL
was removed from them.
Hood Chatham