Add a wasi package command

[brettcannon]

This introduces a wasi package command to Platforms/WASI/ that creates a .tar.xz file as a WASI distribution. This did entail a refactoring to share code between this and the build command. I tried to keep start-up costs down so --help was snappy.

This is very much an MVP for the command itself and the resulting .tar.xz file. Future plans for the command include:

• Getting tests that fail from the distribution passing (including the warning in the REPL about not running from --prefix)
• A --build flag so that wasi package can go from checkout to .tar.xz in one command (including using flags that should be used when building for a distribution)
• A container file that uses the WASI dev container so the packaging requires no toolchain installation beyond Docker/podman

There should probably also be some testing/CI of the distribution at some point.

[vstinner]

Do you plan documenting this new command in Platforms/WASI/README.md or https://devguide.python.org/getting-started/setup-building/#wasi? It would be nice to have a short description of the new package command, of the tarball contents and how it's supposed to be used.

[brettcannon]

Do you plan documenting this new command in Platforms/WASI/README.md

I had not thought about it, but I can plan to do that in another PR to also cover the build command.

[brettcannon]

!buildbot wasi

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @brettcannon for commit d14bb0d 🤖

Results will be shown at:

https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F154510%2Fmerge

The command will test the builders whose names match following regular expression: wasi

The builders matched are:

• wasm32-wasi PR