Draft: Devtools: dedicated devtools opam switch

Context

Currently, the devtools (merlin, ocamlformat, odoc, utop, etc.) are installed in the same opam switch as the core dependencies (Lwt, irmin, data-encoding, etc.). As a result, the dependency version constraints from the devtools affect the update of the core dependencies. E.g., it may not be possible to upgrade a given core dependency because of a conflict with a devtool dependency. (This happens; it is not a theoretical consideration.)

Note that we are only interested in the binaries (and a few runtime files) from these dev dependencies. We do not care about link-time compatibility between our devtools and our core dependencies.

This MR introduces an opam switch dedicated to devtooling. IT IS EXPERIMENTAL.
Here's how to try it out locally.

• Follow the instructions from https://tezos.gitlab.io/introduction/howtoget.html#setting-up-the-development-environment-from-scratch
• Once everything is set-up, run make build-dev-deps. This should create the new switch and install the dependencies there.

Note: you can customise the set of devtools you want to install. Before you run make build-dev-deps, execute the following commands:

• make -C devtools tools-list (sets the default)
• $EDITOR devtools/tools-list (remove unneeded lines, add new ones)

Issues:

• Does not remove tools when you re-run the make target after removing some tools.
• Not tested on many computers, may not work, please let me know.

Companion MR on the opam-repo: opam-repository!331

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR