message: packages/core: prune imports
diff: "prop-types": "^15.7.2",
  "rc-progress": "^3.0.0",
  "react": "^16.12.0",
  - "react-addons-text-content": "0.0.4",
  "react-dom": "^16.12.0",
  "react-helmet": "6.0.0",
  "react-router": "^5.2.0",
  "react-router-dom": "^5.2.0",
  "react-sparklines": "^1.7.0",
  "react-syntax-highlighter": "^12.2.1",
  - "react-use": "^14.2.0",
  - "zen-observable": "^0.8.15"
  + "react-use": "^14.2.0"
  },
  "devDependencies": {
  "@backstage/cli": "^0.1.1-alpha.6",
  "@backstage/test-utils": "^0.1.1-alpha.6",
  - "@backstage/test-utils-core": "^0.1.1-alpha.6",
  "@testing-library/jest-dom": "^5.7.0",
  "@testing-library/react": "^9.3.2",
  "@testing-library/user-event": "^10.2.4",
  