textloc -> loc: rename package, drop Sort_kind, use dune re_export

Rename the shared location/error-infrastructure package to 'loc' (module
Loc), dropping the awkward 'textloc' / 'Textloc' naming. Consumers now
write [Sexpt.Meta], [Sexpt.Error], [Loc.t], etc.

- ocaml-textloc -> ocaml-loc (directory, opam, module).
- Dropped the single-function Sort_kind module; parsers inline the
one-liner at their local Sort.kinded definitions.
- Parser dunes use [(re_export loc)] so downstream consumers don't need
to declare loc in their own dune or opam.
- monopam lint: new [collect_exports] walks META [exports] fields
(dune's re_export metadata) and expands each opam package's
effective dep set with everything its declared deps re-export.
[Fl_split.in_words] replaces the ad-hoc whitespace splitter.
- merlint: drop 'loc' from its dune-project depends -- reaches via
sexpt re_export.
- Consumers (cdm, s3, test files) updated to [Loc.*] naming.

All 360 tests pass (26 loc + 177 xmlt + 60 csvt + 77 sexpt + 20
dune-codec).