This is already a part of the npm package.json spec: https://docs.npmjs.com/files/package.json#local-paths
As long as the local path points to a directory containing a package.json file, this should pull in the contents of that package (along with transitive dependencies), equivalent pulling that package down from an npm server.

This would allow hardlinking local packages to each other, without relying on an external server. This would be useful for a number of reasons:

1) Some packages are proprietary, and public publishing is never an intention. Using an npm server (even a private one) is therefore not a useful abstraction.

2) Large trees of IGs, e.g., if packages are used as a hierarchy (type/region/site, etc), are cumbersome if each version must be published, and linking through a local hierarchy is more logical

3) Local linking allows for a hermetic development/testing environment. Relying on local caching behavior is more brittle/less explicit compared to directly listing desired target.