libutil: generate experimental and deprecated features from data

Currently, a bunch of documentation is generated by embedding parts of it in
the nix executable, getting it out again by running it, and then postprocessing
the output. This is bad, since it creates a pointless dependency of the
documentation on the executable, and also makes documentation generation
impossible when cross-compiling.
Instead, both the code and the documentation should be generated from data, see
https://git.lix.systems/lix-project/lix/issues/292 . Here we start applying
this approach to the experimental and deprecated features, which are done in
one go since the technical implementation is very similar.
Of course, the actual benefits are not realised yet, since the offending
pattern is used in several more places. These will be fixed later.

Change-Id: I4c802052cc7e865c61119a34b8f1063c4decc9cb

doc/manual/src/command-ref/meson.build

@@ -1,37 +1,3 @@
-experimental_features_shortlist_md = custom_target(
-  command : nix_eval_for_docs + [
-    '--expr',
-    'import @INPUT0@ "experimental" "xp" (builtins.fromJSON (builtins.readFile @INPUT1@))',
-  ],
-  input : [
-    '../../generate-features-shortlist.nix',
-    nix_exp_features_json,
-  ],
-  capture : true,
-  output : 'experimental-features-shortlist.md',
-  env : nix_env_for_docs,
-)
-
-dp_features_json = custom_target(
-  command : [nix, '__dump-dp-features'],
-  capture : true,
-  output : 'dp-features.json',
-)
-
-deprecated_features_shortlist_md = custom_target(
-  command : nix_eval_for_docs + [
-    '--expr',
-    'import @INPUT0@ "deprecated" "dp" (builtins.fromJSON (builtins.readFile @INPUT1@))',
-  ],
-  input : [
-    '../../generate-features-shortlist.nix',
-    dp_features_json,
-  ],
-  capture : true,
-  output : 'deprecated-features-shortlist.md',
-  env : nix_env_for_docs,
-)
-
 # Intermediate step for manpage generation.
 # This splorks the output of generate-manpage.nix as JSON,
 # which gets written as a directory tree below.