Allow any version requirement string for searching

The API should allow either a scalar version requirement string, not
just a single version.

It's a breaking change unless single versions are prefixed with "=",
but I'm not sure it's a bad breaking change anyway. Making
index lookup function more like what CPAN clients expect is probably
less surprising.

[kentfredric]

I'd possibly define such a search API that allows either returning one of the following:

• The "newest" version that meets a criteria
• The "oldest" version that meets a criteria (I'll explain)
• A list of packages that meet a criteria.

This is obviously not so straight forward to do with all backends, but there's a lot of things that are possible if one ever adds a MetaCPAN::API backend.

Oldest criteria

Often when you have a larger project, you might need an extra dependency.

That dependency in turn, may itself, require a dependency of something newer than you already have.

Common behaviour at present is to simply update to the latest version, which can have a bunch of negative effects, importantly:

• Pulling more dependencies you don't want/need
• Doing a more radical upgrade than you intended and breaking a lot of code

Hence, I consider it to be useful to perform a "minimal upgrade" to satisfy a dependency, instead of a maximal one.

However, no tools presently make doing that straight forward that I'm aware of.

Side note I don't know how to classify

There also needs to be some sort of way of saying:

needs X 0.5
needs Y 0.6

And having the system intelligently realise that X 0.7 and Y 0.7 are the same distrubition and offer you install choices respectively.

But also when it opts to install only X 0.5 and Y 0.6 which are Different distributions, offer the right install results that way too.

Reason: One common problem I'll see is cpanm installing something to satisfy one dependency , and then later having to downgrade the dependency it just installed in the same dependency tree.

( An example I hit regularly is simplified here: miyagawa/cpanminus#388 )

But I have no idea where in the levels such a query should go.

IIRC, my original search API thoughts were to return a high-to-low ordered list of distributions that match the search criteria. This would leave "oldest" or "newest" logic up to the consumer.