This commit moves the colours module to be a sub-module of the output one.
This makes sense because finding which colour a certain file should be is only
done during output, and (I think) the only places that the `Colours` struct's
fields are ever queried is from the output module.

The only casualty was that the `file_colour` from the filetype module had to
be moved, as determining colours is no longer part of that module - only
determining filetype is. So it now reflects its name!