Add support for constraints, install in topological order

[bollwyvl]

references

• fixes support constraints for transient dependencies? #175

changes

• add micropip.set_constraints
• add constraints to the signatures of install and Transaction
  • per upstream, filter constraints that don't provide enough detail (e.g. Just A Wheel URL)
  • merge specifiers to any requirement if found in the constraints (by canonical name)
• rework install to handle constrained indirect dependencies of packages found in pyodide-lock.json
  • add requested_by to the signatures of many Transaction methods
  • capture the full as-requested Transaction.dependency_graph
  • install packages in topological order based on graph

breaking changes

• hopefully none, as it's opt-in, and only adds keyword-only arguments to the already very finicky install, etc.
  • previously, all packages from pyodide-lock.json were unconditionally installed before any resolved wheels
  • now, all installations are handled the same way, waiting for any dependencies to be installed first
  • further, transactions containing cycles of dependencies will fail

[hoodmane]

Looks nice so far I suppose the next step would be to actually use them.

[ryanking13]

Thanks! I left some comment

[bollwyvl]

Had a thought which might skip all the bookkeeping: still do gather on dependencies of pyodide-lock packages, but then install PyPI wheels first... took a bit to get what's in there working, so will try a sibling PR...

[bollwyvl]

I think #181 is the better way forward, but will keep this open as a draft for reference.

[ryanking13]

Closing in favor of #181