Move the format crates from plugins/<id> to crates/formats/<id> #443

Part of #442. Since #437 the server links in every format, so plugins/ is now just a second directory of workspace crates. The format crates move under crates/, to crates/formats/<id>/. Each directory keeps the format id as its name. The crate names (forge-format-nuget, forge-format-raw) and their code stay the same.

Change

• Run git mv plugins/<id> crates/formats/<id> for every crate in plugins/ at the time: nuget, plus raw once Raw repositories as a built-in format plugin over a host file store #441 has merged. Move plugins/README.md to crates/formats/README.md. plugins/ is then gone.
• Fix the paths that depend on directory depth:
  • the workspace members in Cargo.toml;
  • the path dependencies on the format crates in crates/core, crates/e2e and crates/server (../../plugins/nuget becomes ../formats/nuget);
  • the format crates' own dependency on forge-plugin-api (../../crates/plugin-api becomes ../../plugin-api);
  • tests/parse.rs of NuGet, which reads the fixtures as CARGO_MANIFEST_DIR/../../fixtures/nuget and is now one level deeper.
• Fix the tooling that names plugins. In each place below, crates already covers the new location:
  • the Rust paths in .github/scripts/ci-changes.sh;
  • the touches check in .github/workflows/codeql.yml;
  • the comment in .github/codeql/codeql-config.yml;
  • /plugins/*/target in .gitignore and .dockerignore.
• Rewrite the AGENTS.md section Formats: every format plugin lives under plugins/ as every format lives under crates/formats/:
  • a format is a crate in crates/formats/<id>/;
  • it is never a crate elsewhere under crates/ (no crates/format-<id>) and never a module of the server;
  • Docker / OCI moves to crates/formats/docker, and raw to crates/formats/raw if Raw repositories as a built-in format plugin over a host file store #441 has not merged yet.
• Update every other mention of plugins/<id>:
  • README.md: the crate table and Every format is a plugin in plugins/;
  • DEVELOPMENT.md: the layout and the CI path tables;
  • docs/format-plugins.md and crates/formats/README.md;
  • the module docs of forge-plugin, forge-plugin-api, forge-core and forge-server;
  • the expect messages that name plugins/nuget/plugin.toml;
  • fixtures/nuget/make_fixtures.py.

No changelog fragment is needed, since nothing changes for anyone running Forge.

Acceptance criteria

• plugins/ does not exist, and crates/formats/<id>/ holds every format crate it used to hold.
• cargo clippy --workspace --all-targets -- -D warnings and cargo test --workspace pass, including the NuGet fixture tests.
• git grep finds no path into the old plugins/ directory outside CHANGELOG.md and docs/changes/.
• A change under crates/formats/ still starts the Rust jobs of CI and of CodeQL.
• AGENTS.md, README.md and DEVELOPMENT.md name crates/formats/<id> as the place for a format.

Start after #440 and #441 merge. Both rewrite or add files under plugins/.