We introduce a new package structure for Coq:

- `coq-core`: Coq's OCaml tools code and plugins
- `coq-stdlib`: Coq's stdlib [.vo files]
- `coq`: meta-package that pulls `coq-{core,stdlib}`

This has several advantages, in particular it allows to install Coq
without the stdlib which is useful in several scenarios, it also open
the door towards a versioning of the stdlib at the package level.

The main user-visible change is that Coq's ML development files now
live in `$lib/coq-core`, for compatibility in the regular build we
install a symlink and support both setups for a while.

Note that plugin developers and even `coq_makefile` should actually
rely on `ocamlfind` to locate Coq's OCaml libs as to be more robust.

There is a transient state where we actually look for both
`$coqlib/plugins` and `$coqlib/../coq-core/plugins` as to support
the non-ocamlfind plus custom variables.

This will be much improved once #13617 is merged (which requires this
PR first), then, we will introduce a `coq.boot` library so finally
`coqdep`, `coqchk`, etc... can share the same path setup code.

IMHO the plan should work fine.

Also pass `--output-sync` on the CI, as suggested in
https://github.com/coq/coq/pull/12653#issuecomment-696226093, to protect
against this failure mode.

Fixes #13062

Previously, when either the 'before' or 'after' had no files, we'd get
an uncaught exception:
```
Traceback (most recent call last):
  File "/home/jgross/Documents/repos/coq/tools/make-both-time-files.py", line 16, in <module>
    table = make_diff_table_string(left_dict, right_dict, sort_by=args.sort_by, include_mem=args.include_mem, sort_by_mem=args.sort_by_mem)
  File "/home/jgross/Documents/repos/coq/tools/TimeFileMaker.py", line 391, in make_diff_table_string
    right_peak = max(v.get(MEM_KEY, 0) for v in right_dict.values())
ValueError: max() arg is an empty sequence
```

Fixes #12387

This, for example, improves the CI display, so that `$ echo
'end:coq.test'` does not appear on the same line as the end of the
timing table.

The Python scripts now support `--no-include-mem` to turn it off, and
also support `--sort-by-mem`.

Closes #11575

- Add a `--fuzz` option to `make-both-single-timing-files.py`

  Passing `--fuzz=N` allows differences in character locations of up to
  `N` characters when matching lines in per-line timing diffs.

  The corresponding variable for `coq_makefile` is `TIMING_FUZZ=N`.

  See also the discussion at
  https://github.com/coq/coq/pull/11076#pullrequestreview-324791139

- Allow passing `--real` to per-file timing scripts and `--user` to
  per-line timing script.

  This allows easily comparing real times instead of user ones (or vice
  versa).

- Support `TIMING_SORT_BY` and `TIMING_FUZZ` in Coq's own build

- We also now use argparse rather than a hand-rolled argument parser;
  there were getting to be too many combinations of options.

- Fix the ordering of columns in Coq's build system; this is the
  equivalent of #8167 for Coq's build system.

Fixes #11301
Supersedes / closes #11022
Supersedes / closes #11230

Reviewed-by: ppedrot

Closes #10491

We re-add the header in doc/tools/coqrst/notations/fontsupport.py
which was removed by accident in 1a9c769ed363ee2f2784e7252af72e6c1e2fbcc6

The fontsupport script itself has been kept for reference, however it
is not involved by any build target as of today.

This should have been running already, but it was forgotten in #9872

This fixes #9767 by silently ignoring input lines which are not valid
UTF-8.  We hereby assume that all file paths are valid UTF-8.

We also now actually test both python2 and python3 on the CI.