declared non-SPDX license shoud result in text attachment

[jkowalleck]

in python packaging, the metadata License: field(s) is either a SPDX id/expression, or it is a license text.

current implementation causes it to be a declared license name, instead of a license text.
this needs to be fixed. this does not affect concluded licenses!

expected outcome:

• license name is Declared license of <package>
• license text is attached
  • content is the declared license
  • content type: text/plain
  • could be base64-encoded, though this is not needed - as per all relevant specs the text is expected to be UTF8 string already.

this is partially related to #570

see https://packaging.python.org/en/latest/specifications/core-metadata/#license

Text indicating the license covering the distribution where the license is not a selection from the “License” Trove classifiers. See “Classifier” below. This field may also be used to specify a particular version of a license which is named via the Classifier field, or to indicate a variation or exception to such a license.

[jkowalleck]

have a reproducible example with NumPy -- they have extende dlicene texts in the wheel metadata.