Add compatibility for deprecated python_implementation marker

[charliermarsh]

Summary

Like platform.python_implementation, we should support the python_implementation "alias" for platform_python_implementation.

Closes #1906.

Test Plan

❯ cargo run pip install "pynacl==1.4.0"
    Finished dev [unoptimized + debuginfo] target(s) in 7.02s
     Running `target/debug/uv pip install pynacl==1.4.0`
Resolved 4 packages in 9ms
   Built pynacl==1.4.0                                                                                                                                                                                                                        Downloaded 1 package in 31.51s
Installed 4 packages in 3ms
 + cffi==1.16.0
 + pycparser==2.21
 + pynacl==1.4.0
 + six==1.16.0