Skipped classes can be loaded, activated and displayed on plugin admin page

[Ryan J Ollos]

Issue was found when making changes to ​trac-github plugin. The entry_points configuration for the plugin was changed to:

    extras_require={'oauth': ['requests_oauthlib >= 0.5']},
    entry_points={'trac.plugins': [
        'github.browser = tracext.github:GitHubBrowser',
        'github.loginmodule = tracext.github:GitHubLoginModule[oauth]',
        'github.postcommithook = tracext.github:GitHubPostCommitHook',
    ]},

The GitHubLoginModule still shows on the Plugins admin page when the dependency is not satisfied. The component can be activated when requests-oauthlib is not installed even though it was claimed to be skipped during environment startup.

02:22:27 AM Trac[loader] DEBUG: Skipping "github.auth = tracext.github:GitHubLoginModule [oauth]": ("DistributionNotFound: requests-oauthlib>=0.5" not found)
02:22:27 AM Trac[loader] DEBUG: Loading github.browser from /home/user/Workspace/trac-github-dev/pve/lib/python2.7/site-packages/trac_github-2.1.3-py2.7.egg
02:22:27 AM Trac[loader] DEBUG: Loading github.postcommithook from /home/user/Workspace/trac-github-dev/pve/lib/python2.7/site-packages/trac_github-2.1.3-py2.7.egg

The issue seems to be that since the dependency is a ​method-scoped import, the GitHubLoginModule class is added to the Component registry when other classes in the module are loaded: tags/trac-1.0.8/trac/loader.py@:68#L36​. Primarily it would be nice to not show the Component on the plugin admin page when it is claimed to be skipped.

I don't know if there's a reasonable solution to this, but I just thought I would raise the issue to see if there are any ideas.