bdist_wheel does not work with multiple --plat-name tags

[pombredanne]

There are times (like right now with macOS bigsur triggering several issues) where having a wheel built with more than one plat-name tag would help and be required. These tag names should end up as a dot-separated platform tag. BUT:

• using bdist_wheel --plat-name a --plat-name b will only consider b and ignore a
• using bdist_wheel --plat-name a.b will create an a_b tag instead of an a.b tag

The only sane ways I found are:

1. rename the wheel with a proper dot-separated tag. This works but the WHEEL Tag is not updated
2. use wheel unpack then update the .dist-info/WHEEL Tag then wheel pack. Here the WHEEL file content and wheel file name are in sync.

We should be able to do better!

I suggest that no transformation be applied to the --plat-name option value OR that at least .dots are not replaced by _underscores.

Possibly here https://github.com/pypa/wheel/blob/master/src/wheel/bdist_wheel.py#L104 or/and here https://github.com/pypa/wheel/blob/master/src/wheel/bdist_wheel.py#L265

These small but important changes would mean that we can allow multiples platforms in a single --plat-name option. This would be the preferred way as the use of --plat-name is not a noob option so having it allow advanced values for proper control should be fine.

Alternatively the bdist_wheel --plat-name option should accept multiple values, either coma-separated as in --plat-name=a,b or with multiple --plat-name a --plat-name b

[pombredanne]

This is likely related to #311
@mdavidsaver @jamadden ping since you were on that one.