Manifest: move data-encoding to its own module and set version constraint to an equality

What

This MR:

• sets the version constraint of data-encoding to an equality;
• moves data-encoding external definitions from manifest/externals.ml to manifest/product_data_encoding.ml.

Why

Now that data-encoding is in tezos/tezos we want to make sure that the version number of the external is the one that was snapshot into tezos/tezos. (Also, it makes it easier to share the version number in the module using variable opam_version in the next MR.)

So that we can then replace those externals with internals, without changing the rest of the code. This was extracted from !12004 (original author: @raphael-proust )

How

By moving the code and letting the compiler guide to places that need to be prefixed by Product_data_encoding. But I just added the open and module aliases that @raphael-proust used in !12004.

Manually testing the MR

make -C manifest
./scripts/update_opam_lock.sh

should return an empty diff.

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