Rename doc/manual{src -> source}

This is needed to avoid this
https://github.com/mesonbuild/meson/issues/13774 when we go back to
making our subproject directory `src`.

doc/manual/src/language/constructs/lookup-path.md

@@ -1,24 +0,0 @@
-# Lookup path
-
-> **Syntax**
->
-> *lookup-path* = `<` *identifier* [ `/` *identifier* ]... `>`
-
-A lookup path is an identifier with an optional path suffix that resolves to a [path value](@docroot@/language/types.md#type-path) if the identifier matches a search path entry in [`builtins.nixPath`](@docroot@/language/builtins.md#builtins-nixPath).
-The algorithm for lookup path resolution is described in the documentation on [`builtins.findFile`](@docroot@/language/builtins.md#builtins-findFile).
-
-> **Example**
->
-> ```nix
-> <nixpkgs>
->```
->
->     /nix/var/nix/profiles/per-user/root/channels/nixpkgs
-
-> **Example**
->
-> ```nix
-> <nixpkgs/nixos>
->```
->
->     /nix/var/nix/profiles/per-user/root/channels/nixpkgs/nixos