angular6 how to import a library in another library?

You will need to build your libs first, see this question Angular 6 CLI -> how to make ng build build project + libraries

If you take a look in the tsconfig.json file, it has been modified by adding paths for the libs - hence you need to build them to populate the dist folder.

"paths": {
  "lib": [
    "dist/lib"
  ],
  "lib2": [
    "dist/lib2"
  ]
}