[RRFC] Allow for use of symlinks to hoist local modules

Motivation

Creating a symlink facility from a dependency declaration in a package.json file would go a long way to improve the experience of developers using npm in a monorepo.

Example

My organization develops our Typescript codebase composed of a client, server, and several common packages in a monorepo. Using npm, if I have a file:<my dependent> dependency declared, without running npm link the file is not symlinked in the node_modules directory. This interferes with a number of popular tools, such as React Native, Typescript, Jest, etc when working with monorepos and can make configuration for those tools a headache and reduces the experience of developers working in monorepos.

How

Current Behaviour

Currently using file: in a dependency declaration without running npm link does not create a symlinked local module in the node_modules directory. Consider the following package structure:

- client
    - depends on: common-a, common-b 
- server
    - depends on: common-a
- common-a
- common-b

All modules are written in Typescript. If I run npm install in the necessary order (or with a management tool such as Lerna), the installs will complete successfully. However if I compile common-a and then compile server, the compiler would not be able to find the type definitions of common-a because that module does not exist in the server node_modules directory.

Desired Behaviour

Another dependency specifier could be added (link: would maintain yarn compatibility) to indicate when a true symlink should be created for the dependent local package in the parent node_modules directory. Then tools would be able to find the local module in node_modules as expected.

Willing to implement, will just need guidance on what the relevant parts of the codebase would be.

No files, tests, or entry points are named in the issue. Begin by locating npm’s handling of file: dependencies and related link/install behavior; done means a declared link: dependency creates a true symlink in the parent node_modules and works for the described monorepo.