Exposes packages to constraints (#5555)

**What's the problem this PR addresses?**

Constraints can only operate on the workspaces' listed dependencies, but
have no idea about the shape of the global dependency tree. In general
it's enough, but I wanted to write a constraint autofixing missing peer
dependencies, and this wasn't possible without knowing the dependencies
of our dependencies.

**How did you fix it?**

I added a concept of "package" to the constraint API. Some of their
characteristics:

- They are based on the resolved dependency tree; as a result, there is
no direct concept of devDependency / dependency (just like in the core
`Package` type).

- They somewhat look like the `Package` type, but with fewer information
(just the dependencies & peer dependencies, at the moment), and with a
slightly different format (no need for serialization, so it directly
references other data structure rather than go through string hashes).

- They don't provide any autofix (or even reporting) capability at the
moment. Perhaps later I'll add an `error` API, like regular
dependencies, but that'll probably require to enhance them so they are
able to access the packages' filesystems, so it'll have to be a
follow-up.

Testing is currently lacking ... like the rest of the JS constraints
engine 😶‍🌫️

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Author: arcanis

.pnp.cjs

@@ -0,0 +1,25 @@
+releases:
+  "@yarnpkg/cli": major
+  "@yarnpkg/plugin-constraints": major
+  "@yarnpkg/types": major
+
+declined:
+  - "@yarnpkg/eslint-config"
+  - "@yarnpkg/plugin-compat"
+  - "@yarnpkg/plugin-dlx"
+  - "@yarnpkg/plugin-essentials"
+  - "@yarnpkg/plugin-init"
+  - "@yarnpkg/plugin-interactive-tools"
+  - "@yarnpkg/plugin-nm"
+  - "@yarnpkg/plugin-npm-cli"
+  - "@yarnpkg/plugin-pack"
+  - "@yarnpkg/plugin-patch"
+  - "@yarnpkg/plugin-pnp"
+  - "@yarnpkg/plugin-pnpm"
+  - "@yarnpkg/plugin-stage"
+  - "@yarnpkg/plugin-typescript"
+  - "@yarnpkg/plugin-version"
+  - "@yarnpkg/plugin-workspace-tools"
+  - "@yarnpkg/builder"
+  - "@yarnpkg/core"
+  - "@yarnpkg/doctor"