merge radicle-source into radicle-surf

Initially, radicle-source was envisioned to be another layer on top
of radicle-surf, however, it ended up duplicating a lot of concepts.

We consolidate them by merging radicle-source into radicle-surf.
This meant removing modules branch.rs, tag.rs, oid.rs and error.rs.

.github/workflows/ci.yaml

@@ -87,8 +87,8 @@ jobs:
       # The 'main' branch is needed for radicle-surf vcs::git::tests::test_submodule_failure,
       # however actions/checkout uses the default branch 'master' at this moment.
       # We will improve the test soon to remove this need.
-      - run: git branch main
-      - run: git branch -u origin/main main
+      # - run: git branch main
+      # - run: git branch -u origin/main main
       - uses: actions-rs/toolchain@v1
         with:
           profile: minimal