docs(linker): Use breadcrumbs in peer dependency warnings (#4720)

**Summary**

@gaearon called Yarn out for better peer dependency error messages so this is the first step to make
them more useful and informative.

**Test plan**

Use a repo like `https://github.com/edmorley/yarn-unmet-peerdependency-testcase` to have peer dependency
warnings and observe they now show the path for the package missing the peer dependency. This should
allow people to investigate these errors in an easier fashion.

```
warning "neutrino-preset-airbnb-base > eslint-config-airbnb-base@12.0.2" has unmet peer dependency "eslint@^4.8.0".
warning "neutrino-preset-airbnb-base > eslint-plugin-import@2.7.0" has unmet peer dependency "eslint@2.x - 4.x".
```

src/package-linker.js

@@ -468,7 +468,7 @@ export default class PackageLinker {
 
  let peerError = 'unmetPeer';
  let resolvedLevelDistance = Infinity;
- let resolvedPeerPkgPattern;
+ let resolvedPeerPkg;
  for (const peerPkg of peerPkgs) {
  const peerPkgRef = peerPkg._reference;
  if (!(peerPkgRef && peerPkgRef.patterns)) {
@@ -478,25 +478,31 @@ export default class PackageLinker {
  if (isFinite(levelDistance) && levelDistance < resolvedLevelDistance) {
  if (this._satisfiesPeerDependency(range, peerPkgRef.version)) {
  resolvedLevelDistance = levelDistance;
- resolvedPeerPkgPattern = peerPkgRef.patterns;
- this.reporter.verbose(
- this.reporter.lang(
- 'selectedPeer',
- `${pkg.name}@${pkg.version}`,
- `${peerDepName}@${peerPkgRef.version}`,
- peerPkgRef.level,
- ),
- );
+ resolvedPeerPkg = peerPkgRef;
  } else {
  peerError = 'incorrectPeer';
  }
  }
  }
 
- if (resolvedPeerPkgPattern) {
- ref.addDependencies(resolvedPeerPkgPattern);
+ if (resolvedPeerPkg) {
+ ref.addDependencies(resolvedPeerPkg.patterns);
+ this.reporter.verbose(
+ this.reporter.lang(
+ 'selectedPeer',
+ `${pkg.name}@${pkg.version}`,
+ `${peerDepName}@${resolvedPeerPkg.version}`,
+ resolvedPeerPkg.level,
+ ),
+ );
  } else {
- this.reporter.warn(this.reporter.lang(peerError, `${pkg.name}@${pkg.version}`, `${peerDepName}@${range}`));
+ this.reporter.warn(
+ this.reporter.lang(
+ peerError,
+ `${refTree.join(' > ')} > ${pkg.name}@${pkg.version}`,
+ `${peerDepName}@${range}`,
+ ),
+ );
  }
  }
  }