Fix @ parsing in versions

[elcritch]

Dependency versions would get messed up with @ being added to the pkg name in some cases.

[Araq]

Why the heck would @ be part of a package name?

[elcritch]

Why the heck would @ be part of a package name?

Come to think of it shouldn't. I thought maybe it was from some requires format, but that's not a thing.

Hmmm, I'm digging into where the @ is coming from. Probably an errant usage of the pretty print format for a specific version.

[elcritch]

That makes more sense. An old package of mine had an @ in a requires line in an old release. Probably from copying a nimble install pkg@#head or futzing with Nimble.

Now the addRelease logic can handle bad Nimble releases by skipping them. Surprising we haven't needed that yet.

[Araq]

1. It's weirdly specific. There are other invalid chars, I'm sure.
2. I think an Option would work better here rather than an exception, but I don't know for sure.