model_utils: limit dependency to `model`.

Also update tools importing `model_utils`

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
nitdoc: limit dependency to `model`

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
nitdoc: clean output_dir option selection

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
nitdoc: extract NitdocContext from Nitdoc generation

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>