yarn-plugin: rewrite using reduceDependency hook

Switching to using the reduceDependency hook
simplifies the plugin, since we can do all the
work in one go. It also means that the yarn.lock
file doesn't need to change when adding and
removing the yarn plugin, which avoids
unintentionally unlocking packages versions.

As part of this change, the descriptor emitted by
the plugin has been changed from
`npm:<manifest-version>` to
`npm:^<manifest-version>`, to match the range
emitted when packing packages. This ensures good
interoperability with the `backstage-cli
build-workspace`, which packs packages into a new
workspace and copies yarn.lock across.

Signed-off-by: MT Lewis <mtlewis@users.noreply.github.com>

packages/yarn-plugin/package.json

@@ -41,6 +41,7 @@
     "@backstage/backend-test-utils": "workspace:^",
     "@backstage/cli": "workspace:^",
     "@yarnpkg/builder": "^4.0.0",
+    "fs-extra": "^11.2.0",
     "nodemon": "^3.0.1"
   }
 }