Add WHEEL metadata to tag meta import compatible wheels

[qwcode]

Currently, it's entirely up to users to decide whether or not a particular wheel is compatible with zipimport.

Wheel publishers should have a way to indicate that there wheel is import compatible, and then tools like distlib.mount can reject all wheel's without such a marker as incompatible.

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migrated from https://bitbucket.org/pypa/pypi-metadata-formats/issues/17/add-wheel-metadata-to-tag-meta-import