Manifest: remove tezos- exception for external packages

Context

Read the FIXME :)

Before we fully converted all packages to the manifest, internal packages that were not converted were declared as external dependencies when we needed to depend on them. But this broke scripts/opam-release.sh. To fix it we added a heuristic that considered that any external package named tezos-* or octez-* were actually internal for the purpose of scripts/opam-release.sh. But we had to add exceptions to this list, because some packages named tezos-* were really external, like tezos-rust-lib.

Now that all packages are fully converted, we don't need this heuristic anymore.

Manually testing the MR

Check that the released packages are the same before and after this MR:

git checkout master
scripts/opam-release.sh 14.0 Makefile
git checkout romain-manifest-fixme
scripts/opam-release.sh 15.0 Makefile
cd opam-repository
git diff 14.0 --stat

The last command should only show renamings, no differences in the files.

Checklist

• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR