Remove menus of all conda envs during uninstall

[mbargull]

This makes the uninstaller remove not only the root environment's menus but also the menus from all other environments (if possible).
It tries to get the list of environments via the conda.cli.python_api.
An alternative implementation is provided, which uses libconda.config.envs_dirs and code copied from conda.misc.list_prefixes to get the list of environments. Since conda.misc.list_prefixes should be considered internal/subject to change, I feel more comfortable using conda.cli.python_api instead, which is thus the active implementation in this PR.

Use case:
An user creates new environments with packages containing menuinst menus. The user then uninstalls the whole constructor-created installation without explicitly removing those environments beforehand. Prior to this PR the menu entries created for packages outside the root environment would persist after the uninstallation.

[msarahan]

Thanks for this helpful change, @mbargull. Merging.

[github-actions]

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Thanks!