cli-node, cli: move yarnPlugin and SuccessCache to cli-node

Move `getHasYarnPlugin` and `SuccessCache` from `@backstage/cli` internal
modules to `@backstage/cli-node` as public exports, making them available
for reuse by other CLI tooling.

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

packages/cli-node/package.json

@@ -39,6 +39,7 @@
     "@yarnpkg/parsers": "^3.0.0",
     "fs-extra": "^11.2.0",
     "semver": "^7.5.3",
+    "yaml": "^2.0.0",
     "zod": "^3.25.76"
   },
   "devDependencies": {