bug: license file must be SPDX

[baszalmstra]

When a pyproject.toml contains:

license = { file = "LICENSE.md" }

the python backend expects the filename to be a valid SPDX name. E.g.

Error:   × failed to solve requirements of environment 'default' for platform 'win-64'
  ├─▶   × failed to extract metadata for package 'parcels'
  │
  ╰─▶   × Failed to parse recipe

      Error:
        × failed to parse SPDX license: 'LICENSE.md', because: unknown term
        │ See <https://spdx.org/licenses> for the list of valid licenses.
        │ Use 'LicenseRef-<MyLicense>' if you are using a custom license.
        [Failed to read contents for label `here` (offset: 540, length: 10): OutOfBounds]

Reproducible example:

https://github.com/baszalmstra/pixi-build-python-broken-license