Build Tooling: Skip package for build if package.json unreadable

[aduth]

This pull request seeks to resolve an issue where npm run dev can log errors for missing packages, typically when changing branches to an older branch:

⇒ npm run dev

> [email protected] dev:packages /Users/andrew/Documents/Code/gutenberg
> node ./bin/packages/watch.js

internal/modules/cjs/loader.js:797
    throw err;
    ^

Error: Cannot find module '/Users/andrew/Documents/Code/gutenberg/packages/foo/package.json'
...

The problem is that when switching to older branches where a package may not have yet been implemented, the package may be left lingering in the working directory, presumably because its built contents are considered in .gitignore.

In these cases, it should be safe to simply ignore the package. The implementation here generalizes this to ignore any directory in packages for which the package.json file cannot be read.

Testing Instructions:

Verify that an empty directory under packages does not cause a build failure on npm run dev:packages:

1. mkdir packages/foo
2. npm run dev:packages
3. Note that "-> Watching for changes..." is logged, changes to valid packages still cause a rebuild

[aduth]

This should fix a (rather frustrating) issue where rebuilds don't happen at all if the initial error occurs where an extra (non-package) directory was found.