Get rid of special-casing of system.path in the manual generation
Improve option description
Allow options to be marked as "internal"

This means they're not for end users.  Currently they're filtered from
the manual, but we could include them in a separate section.
Manual: Don't show obsolete/deprecated options
Fix style
Slightly improve option type error messages
Allow modules to define warnings to be shown during evaluation
Add a way to define obsolete options that have no replacement
rename.nix: Don't use splitString

It's very slow.
Do show aliases in the manual.
Manual: Don't depend on the absolute path of modules

This ensures that the manual has the same store path regardless of the
directory in which Nixpkgs is stored, and thus can be fetched from the
channel.
Allow literalExample in subexpressions of examples
Manual: Check that we don't depend on the Nixpkgs location
Remove dependencies on the Nixpkgs location