Packages::Search: Implement find_similar

Suggested by Enrico Zini.

For a given <package>, search for P<package>,
and then extract all the terms from the document
you find (you could also implement this part by
just looking up the description and applying the
same algorithms to it as in parse-packages, haven't
benchmarked that against each other).

With these terms then make a OR search. Ignore all
results <package>, since these are obvious ;)
The first few matches of the rest are usually
packages very similar to the one we started with.