Clarify/Fix monorepo package resolution

[CxRes]

🐛 bug report

Links to different packages in a monorepo do not resolve unless pointing directly to a file.
At least this is a case of documentation, though I suspect it is a bug.

🎛 Configuration (.babelrc, package.json, cli command)

Consider two packages 'a' and 'b':

a/package.json:

{
  "source": "./src/index.pug"
}

b/package.json:

{
  "source": "./src/index.pug"
}

😯 Current Behavior

Run:

parcel a otherpackages

With a file b/src/index.pug, none of the following are resolved:
a/src/index.pug:

a(href="/b/")
a(href="b/")
a(href="../../b")

But this works just fine:
a/index.pug:

a(href="/b/src/index.pug")

I can also run, but shouldn't need to:

parcel a b otherpackages

🤔 Expected Behavior

When the source is defined in package.json, Parcel should be able to resolve links to packages and build its contents as part of the build.

🔦 Context

Hard coding path to file is bad, as it breaks the decoupling between the packages. Changing the source in a package b will cause a build to break unless I now modify package a as well.

💻 Code Sample

🌍 Your Environment

Irrelevant in this instance!

Software	Version(s)
Parcel	2.7.0
Node	18.8.0
npm/Yarn	8.13.2
Operating System	Windows 10 x64

[CxRes]

Incidentally, installing 'b' in 'a' and pointing to the npm package name does not work also!