- Don't accept leading zeros in Version or VersionRange
- Fixes https://github.com/haskell/cabal/issues/163
  (pkg-config uses a more general version scheme)
- Also resolves https://github.com/haskell/cabal/issues/5138
- `PkgconfigVersion` is compared with `rpmvercmp`
- `PkgconfigVersionRange` is subset of `VersionRange`
    - with `cabal-version` before 3.0 it's parsed like `VersionRange`,
      where version digits are arbitrary integral (leading spaces
      allowed)
    - starting from cabal spec 3.0 `== x.y.*` and `^>=`
      (and set `{ ..  }`) are disallowed.
      Yet, the version literals syntax is relaxed to accept
      alphanumerical + `-.` strings. E.g. openssl's `1.1.0h` is
      accepted.
- Lax `PkgconfigVersion` parser is also used to parse
  `pkg-config --modversion` output.