Add support for scoped package names

[lionello]

Currently, a package name like @lionello/blah will cause a very obscure error during symlinking of the final outputs:

building
installing
Traceback (most recent call last):
  File "/nix/store/2rsm2ng603mxh07ca0v8lwmgxwxvdj8r-link-bin-outputs.py", line 64, in <module>
    os.symlink(fin, fout)
OSError: [Errno 2] No such file or directory
patching script interpreter paths in /nix/store/4m17q31z4zixhfn548nq936kdak3g70m-lionello-blah-1.0.3-lib/node_modules/lionello-blah/bin/cli
/nix/store/4m17q31z4zixhfn548nq936kdak3g70m-lionello-blah-1.0.3-lib/node_modules/lionello-blah/bin/cli: interpreter directive changed from "/usr/bin/env node" to "/nix/store/5s22c7ni2wd1akdsmiq3h73cg8mc7z5z-nodejs-10.18.1/bin/node"
builder for '/nix/store/fy4yw2asvx5lv5x7gcx96sadqdljgmsf-lionello-blah-1.0.3.drv' failed with exit code 1
error: build of '/nix/store/fy4yw2asvx5lv5x7gcx96sadqdljgmsf-lionello-blah-1.0.3.drv' failed

This is because link-bin-outputs.py is being invoked with @lionello/blah as lib_out, which doesn't exist.