Grafana Data: Use package.json exports for internal code (#102696)

* refactor(frontend): rename all @grafana/data/src imports to @grafana/data

* feat(grafana-data): introduce internal entrypoint for sharing code only with grafana

* feat(grafana-data): add test entrypoint for data test utils usage in core

* refactor(frontend): update import paths to use grafana/data exports entrypoints

* docs(grafana-data): update comment in internal/index.ts

* refactor(frontend): prefer public namespaced exports over re-exporting via internal

* chore(frontend): fix a couple more weird paths that typescript complains about

packages/grafana-data/package.json

@@ -15,6 +15,26 @@
   },
   "main": "src/index.ts",
   "types": "src/index.ts",
+  "module": "src/index.ts",
+  "exports": {
+    "./package.json": "./package.json",
+    ".": {
+      "import": "./src/index.ts",
+      "require": "./src/index.ts"
+    },
+    "./internal": {
+      "import": "./src/internal/index.ts",
+      "require": "./src/internal/index.ts"
+    },
+    "./unstable": {
+      "import": "./src/unstable.ts",
+      "require": "./src/unstable.ts"
+    },
+    "./test": {
+      "import": "./test/index.ts",
+      "require": "./test/index.ts"
+    }
+  },
   "publishConfig": {
     "main": "./dist/cjs/index.cjs",
     "module": "./dist/esm/index.mjs",