ENH improvements to pyodide-build

[hoodmane]

Description

More improvements to pyodide-build. This includes:

1. some tuneups to pywasmcross extracted from Update to Scipy 1.7.3 #2065
2. a --continue flag that allows skipping parts of the package build in buildpkg for debugging the build
3. better handling of skipping already built packages in buildall -- in particular, it will now rebuild a package if a package that it depends on is being rebuilt, and the messages printed are a bit clearer.
4. a --force flag to force a rebuild

Checklists

• Add a CHANGELOG entry
• Add / update tests
• Update documentation

[rth]

A few comments, otherwise LGTM. Thanks!