wp-build: Update package detection to ignore empty folders

[aduth]

What?

Updates the build script to ignore package folders which don't contain a package.json.

Why?

When changing from a branch which introduces a new package to trunk, the package folder is not removed from the local filesystem, only the files within it. This causes npm run build to fail because it detects the folder, but cannot read the package.json:

❌ Build failed: Error: ENOENT: no such file or directory, open '/gutenberg/packages/foo/package.json'

How?

Updates the package logic to only detect folders which contain a package.json file.

Testing Instructions

Verify that adding an empty folder in packages/ does not cause the build to fail as it does on trunk:

mkdir packages/foo
npm run build

Observe that the number of packages built is the same between trunk and this branch.

[youknowriad]

It did happen to me several times. Thanks for fixing it

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Co-authored-by: aduth <[email protected]>
Co-authored-by: youknowriad <[email protected]>

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