shared libproj; tests and rebuild for pyproj

[MMesch]

Template B: Checklist for updating a package

• ⚠️ Bump build number if the version remains unchanged
• [] Or reset build number to 0 if updating the package to a newer version

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PR Formatting

• PR title follows format: Add [package-name] or Update [package-name] to [version] (unsure since this isn't a package update)
• PR description includes:
  • Version being added/updated
  • Any special build considerations or patches applied

Package Details

• Package Name: libproj, pyproj
• Version: unchanged versions

Build Notes

• I build proj as shared library in this PR. This seems to reduce size from 118Mb (pyproj) to 1.8MB (pyproj) + 4.6MB (libproj) since pyproj otherwise embeds and duplicates much of proj's functionality across it's ~10 .so files. A shared build therefore seems to be the right call here.
• I deactivated the SONAME in the build as we did in PR #6001, for the same reason that the wasm linker identifies libraries by full name without understanding of semantic version or deduplication of symlinks. This means that two libraries loading the same proj in two different ways, end up with two instances of it which is usually unintended and throws exceptions if each instance handles its own state.
• I added a set of additional tests to this.
• I also remove pip from the host build dependencies since it seems it is already provided by cross_python. The additional pip threw a build error (locally on my machine), likely because of duplication. Removing it seems to work fine.

[IsabelParedes]

Thank you!