Infer FaaS runtime from interpreter constraints, when unambiguous

[huonw]

This allows the runtime argument to python_aws_lambda_function, python_aws_lambda_layer and python_google_cloud_function to be inferred from the relevant interpreter constraints, when they cover only one major/minor version. For instance, having ==3.9.* will infer runtime="python3.9" for AWS Lambda.

The inference is powered by checking for two patterns of interpreter constraints that limit to a single major version: equality ==3.9.* (implies 3.9) and range >=3.10,<3.11 (implies 3.10). This inference doesn't always work: when it doesn't work, the user gets an error message to clarify by providing the runtime field explicitly. Failure cases:

• if the interpreter constraints are too wide (for instance, >=3.7,<3.9 covering 2 versions, or >=3.11 that'll eventually include many versions), we can't be sure which is meant
• if the interpreter constraints limit the patch versions (for instance, ==3.8.9 matching a specific version, or ==3.9.*,!=3.9.10 excluding one), we can't be sure the cloud environment runs that version, so inferring the runtime would be misleading
• if the interpreter constraints are non-obvious (for instance, >=3.7,<3.10,!=3.9.* is technically 3.8 only), we don't try too hard to handle it. We can expand the inference if required in future.

For instance, if one has set [python].interpreter_constraints = ["==3.9.*"] in pants.toml, one can build a lambda artefact like (and similarly for a GCF artifact):

python_sources()
python_aws_lambda_function(name="func", entry_point="./foo.py:handler")

This is the final piece* of my work to improve the FaaS backends in Pants 2.18:

• using the simpler "zip" layout as recommended by AWS and GCF, deprecating Lambdex (Construct Python serverless artefacts (AWS Lambda etc.) in expected layout, without PEX #18879)
• support for AWS Lambda layers (Allow building AWS Lambda Layers #18880)
• Pants-provided complete platforms JSON files* when specifying a known runtime (Include appropriate complete_platforms for serverless/FaaS environments runtime #18195)
• this PR, inferring the runtime from ICs, when unambiguous (including using the new Pants-provided complete platform when available) (Infer appropriate FaaS runtime from interpreter constraints #19304)

(* The fixed complete platform files are currently only provided for AWS Lambda, not GCF. #18195.)

The commits are individually reviewable.

Fixes #19304

[benjyw]

I like it!

[huonw]

Final commit just cleans out the remnants of the old approach that I had missed earlier (including updating the comment).