Use pkg_resources instead of pip internal methods

[derekbekoe]

pip is moving it's internal methods to be private in the future so we should no longer rely on them.
https://pip.pypa.io/en/latest/user_guide/#using-pip-from-your-program

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[azuresdkci]

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[derekbekoe]

Marked as DNM whilst I do some verification with the different installers.

[derekbekoe]

Verified.