Reconsider excluding .meta files from indexed package folders

We currently index all files in the C# solution. We extend the list of indexed files to include read only packages, either in `Library/PackageCache` or included via `file:` references. This means we index `.meta` files for packages, but not for the project (we also don't index any files in `Assets` other than what is included in a `.csproj`).

If you search for a file name that lives in the project, the results will show the appropriate `.cs` file. If you search for a file name the lives in a package, the results will show `.cs` and `.cs.meta`.

(These files can be hidden by disabling the "UnityYaml" file type. Find out why the Search Everywhere popup is showing the ID of the file type and not the "Unity asset file" description)

We rarely need to have `.meta` files in the result - if we did, we should be consistent and index all `.meta` files in the `Assets` folder, too. We should consider excluding `.meta` files when adding the package folders to the index.

Start by locating the indexing logic that adds Library/PackageCache and file:-referenced package folders to the C# solution, then inspect how UnityYaml files are represented in the Search Everywhere popup. Done means package .meta files are excluded from search results while the existing project-file indexing behavior remains unchanged.

Written by the indexing model from the issue text.