Tweaks to new extension code

[eslavich]

Some fixes for issues that were uncovered while testing converters in asdf-astropy:

• Sort plugins by package name + plugins from the asdf package at the end so that extension packages can override built-in plugins.
• Change AsdfFile.extensions property to return "user extensions" only, that is to say, extensions that were passed in to AsdfFile.__init__ or asdf.open.
• Update docstrings to note where ExtensionProxy is returned.
• Remove some features that allowed referring to extensions by URI. I think I went a little overboard with implementing those before we had a real use case, they can always be revived later.
• Include the extension URI in the ExtensionProxy repr to aid in debugging.

[jdavies-st]

Looks good. 👍