ExtensionInfo, the internal class for storing various bits of data about

an extension (paths, names, metadata, etc.) previously required the use

of a Python EntryPoint in order to retrieve the data. This made unit

tests more annoying than they should have been.
Now, ExtensionInfo's constructor takes the extension class, package

name, and metadata. A new create_from_entrypoint() class method has

been added for using an EntryPoint. The old constructor still accepts

one as well, but will warn about deprecated usage first.
Some unit tests have also been updated to make use of this, and to test

the old behavior (modernizing some of the checks while there).

Djblets and Review Board unit tests pass.
Made use of this with the new, upcoming extension testing support.