Fix extension version

[noelbundick]

Fixes #6441 by pulling from METADATA instead of metadata.json, which was removed in pypa/wheel#195

PEP 427 defines METADATA, so this change should be safe for all versions of wheel

This allows extension authors to use the latest version of wheel and still have their version / metadata visible in az extension list and az extension show

This is an internal-only change and does not modify any user-facing input/output formats

[codecov-io]

Codecov Report

Merging #6555 into dev will not change coverage.
The diff coverage is n/a.

@@         Coverage Diff         @@
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  Coverage    0%      0%           
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  Files       11      11           
  Lines      133     133           
  Branches     9       9           
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  Misses     133     133

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[derekbekoe]

@noelbundick The current implementation in the CLI does a json.load which is quick.

This is the full implementation of the method you're using.
https://github.com/jythontools/wheel/blob/master/wheel/metadata.py#L90-L210
This could be very slow to run and extension metadata is needed in several time-critical places in the CLI.

It may be worth checking if this has a perf. impact on the CLI.

[noelbundick]

@derekbekoe good catch - yeah it's right at 10x slower. Closing this out for now and may resubmit one in the future if I get time to implement something faster