Accept + in PkgconfigVersion

See https://github.com/haskell/cabal/issues/163#issuecomment-585826788

Currently we have

```haskell
instance Parsec PkgconfigVersion where
parsec = PkgconfigVersion . BS8.pack <$> P.munch1 predicate where
predicate c = isAsciiAlphaNum c || c == '.' || c == '-'
```

But I *think* we can accept all non-space, non-comma and non `&|<=>` characters. (Technically we can accept also `&|<=>`, but then the space will be required between the "version range operators" and version) Yet it would be really nice if there were some reference what kind of characters are used out there; underspecification of what pkg-config accepts / recommends is not nice, as we need to be able to combine these version strings into larger grammars