npm 5 install folder without using symlink

Use npm pack + npm install (as suggested by install-local package)

npm pack <path-to-local-package>
npm install <package-version.tgz>

This will effectively copy your local package to node_modules.
Note that this will package only production relevant files (those listed in the files section of your package.json). So, you can install it in a test app under the package own directory. Something like this:

my-package
  package.json
  test
    test-app
      package.json
      node_modules
        my-package

Assuming that test dir is not included in the files in my-package/package.json.

This works the same way with npm 5 and older versions.