Custom eslint rule for disallowing cross-package references

Summary:
Added infra for writing and using custom eslint rules and created a rule for disallowing cross-package file imports. Such imports are anti-pattern and they also break standalone plugin bundling.

We still have a bunch of places where Flipper core references code directly from plugins. I've ignored these places for now and added task T71355623 to revisit them.

Reviewed By: passy

Differential Revision: D22998955

fbshipit-source-id: d04cff8fc115ba1300a7e6830306ec134046e927

desktop/.eslintrc.js

@@ -38,6 +38,7 @@ module.exports = {
     'import',
     'node',
     'react-hooks',
+    'flipper',
   ],
   rules: {
     // disable rules from eslint-config-fbjs
@@ -69,6 +70,7 @@ module.exports = {
     'import/no-unresolved': [2, {commonjs: true, amd: true}],
     'node/no-extraneous-import': [2, {allowModules: builtInModules}],
     'node/no-extraneous-require': [2, {allowModules: builtInModules}],
+    'flipper/no-relative-imports-across-packages': [2],
   },
   settings: {
     'import/resolver': {