feat: eslint rule to check forbidden plugin imports

basically verify-local-dependencies.js but done during linting also in
the 3rd party repositories.

Signed-off-by: Hellgren Heikki <heikki.hellgren@op.fi>

packages/eslint-plugin/src/__fixtures__/monorepo/packages/foo/package.json

@@ -1,5 +1,12 @@
 {
   "name": "@internal/foo",
+  "backstage": {
+    "role": "frontend-plugin"
+  },
+  "files": [
+    "dist",
+    "type-utils"
+  ],
   "dependencies": {
     "@internal/bar": "1.0.0"
   },
@@ -8,9 +15,5 @@
   },
   "peerDependencies": {
     "react": "*"
-  },
-  "files": [
-    "dist",
-    "type-utils"
-  ]
+  }
 }

packages/eslint-plugin/src/no-mixed-plugin-imports.test.ts

@@ -0,0 +1,68 @@
+/*
+ * Copyright 2023 The Backstage Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { RuleTester } from 'eslint';
+import path from 'path';
+import rule from '../rules/no-mixed-plugin-imports';
+
+const RULE = 'no-mixed-plugin-imports';
+const FIXTURE = path.resolve(__dirname, '__fixtures__/monorepo');
+
+const ERR = (
+  sourcePackage: string,
+  sourceRole: string,
+  targetPackage: string,
+  targetRole: string,
+) => ({
+  message: `${sourcePackage} (${sourceRole}) uses forbidden import from ${targetPackage} (${targetRole}).`,
+});
+
+// cwd must be restored
+const origDir = process.cwd();
+afterAll(() => {
+  process.chdir(origDir);
+});
+process.chdir(FIXTURE);
+
+const ruleTester = new RuleTester({
+  parserOptions: {
+    sourceType: 'module',
+    ecmaVersion: 2021,
+  },
+});
+
+ruleTester.run(RULE, rule, {
+  valid: [
+    {
+      code: `import '@internal/inline'`,
+      filename: path.join(FIXTURE, 'packages/bar/src/index.ts'),
+    },
+  ],
+  invalid: [
+    {
+      code: `import '@internal/foo'`,
+      filename: path.join(FIXTURE, 'packages/bar/src/index.ts'),
+      errors: [
+        ERR(
+          '@internal/bar',
+          'frontend-plugin',
+          '@internal/foo',
+          'frontend-plugin',
+        ),
+      ],
+    },
+  ],
+});