Don't require a system-installed Python interpreter for bootstrapping

[brandjon]

Forked from discussion in #7947. On windows, we can have the native launcher take on the responsibilities of the stub script, in particular extracting the runfiles. This means it can use any Python runtime to launch the user code, including a runtime in runfiles.

We should also consider creating a native launcher for non-Windows platforms to do the same, so we don't need the #!/usr/bin/env python shebang.

Finally, we can consider merging the zip file into the launch to create a self-extracting native executable.

[laszlocsomor]

/cc @laszlocsomor

[abergmeier]

We should also consider creating a native launcher for non-Windows platforms to do the same, so we don't need the #!/usr/bin/env python shebang.

Can you write up a proposal for goals and rough ideas?