Configurable watch root

[jondlm]

related to: #4332

↪️ Pull Request

Adds a new --watch-dir CLI flag that allows users to specify a directory higher up than the project root. This allows Parcel in watch mode to detect changes to symlinked dependencies.

This is especially useful for monorepos that have lockfiles for each of the projects and where all projects can't share the same project root for various reasons.

💻 Examples

Suppose we have a project shaped like this:

.
├── package.json
└── packages
    ├── app
    │   ├── index.js              <- imports lib via the symlink in node_modules
    │   ├── node_modules
    │   │   └── lib -> ../../lib
    │   ├── package.json
    │   └── yarn.lock
    └── lib
        ├── index.js
        ├── package.json
        └── yarn.lock

We can run parcel server --watchDir ../.. from within packages/app and changes to packages/lib/index.js will trigger HMR updates, etc.

🚨 Test instructions

I have included an integration test that exercises this new behavior. I confirmed that without the watchDir option Parcel won't detect changes but it does with the proper watch directory set.

✔️ PR Todo

• Added/updated unit tests for this change
• Filled out test instructions (In case there aren't any unit tests)
• Included links to related issues/PRs

[devongovett]

we should probably also update this so it works for one-shot builds

parcel/packages/core/core/src/RequestTracker.js

Line 1217 in e4f0a69

let events = await options.inputFS.getEventsSince(

idk if watchRoot is the most obvious name in that case but not sure about another one. projectRoot is a different thing and affects more stuff so would need to be something different.

[jondlm]

Done! Yeah the name because less intuitive when used in this context :|