Fix dependency-resolution parsing and clean Haddock warning

[oneingan]

This PR cleans up build/doc warnings when building with recent nixpkgs/GHC.

• Avoid partial head in --dependency-resolution parsing; require jenkins:<VERSION> explicitly.
• Update flake inputs/lock to current nixos-unstable + simplify flake-compat usage.
• Relax Cabal bounds to match the newer nixpkgs set.
• Minor parser refactor for optional manifest keys.
• Docs tweak to avoid Haddock's ambiguous Manifest identifier warning.

[oneingan]

Added a real conversion-only mode via --no-deps (no transitive dependency recursion). This matches workflows where deps are pre-resolved externally (e.g. jenkins-plugin-cli --list).

Example:

jenkinsPlugins2nix --no-deps $(awk '{printf " --plugin " $0}' plugins.txt.lock) > plugins.nix

Docs and --help updated accordingly.