Refactor exists logic for plugin provider

[tylerjl]

Pull request acceptance prerequisites:

• Signed the CLA (if not already signed)
• Rebased/up-to-date with base branch
• Tests pass
• Updated CHANGELOG.md with patch notes (if necessary)
• Updated CONTRIBUTORS (if attribution is requested)

As part of an upstream change to support sub-plugins, unexpected files (such as .name) in plugin directories will now cause unexpected behavior, including the inability of Elasticsearch to start up correctly. Historically this module has used a helper file called .name in the root of plugin directories to track the name/version in order to determine when plugins should be updated/installed, so this change updates the provider to use a more friendly method of consuming plugin metadata by leveraging the pre-existing Java properties file.

While the change modifies how the plugin provider works pretty drastically, the tests still work as expected and it should handle the edge cases that I can think of. At the very least, it's a much more tenable solution than dropping arbitrary .name metadata files to try and track plugin state.