Here's an updated patch for package, package-x and tests.

Please comment, I'd like to install it soon-ish.

Notes:

* Converting from plist to alist and back is a hassle, but it gives us 
an opportunity to clear out keys with nil values in 
`package-desc-from-define'.
* Not passing :homepage to `package-desc-from-define' in 
`package-buffer-info' when its value is nil seems hard.
* `package--add-to-archive-contents' tries to retain backward 
compatibility by checking the given vector's length.

Now we just need a package archive that would include homepage information.