Don't check for missing package sources in banner provider

[bioball]

The getLibraryRoots() call can be expensive, and can be a cause of slowdown when viewing a file.

Currently, we use this method call within PklSyncProjectNotificationProvider, which is queried for every Pkl module.

This has resulted in warnings from IntelliJ about a slow operation that is being run in the UI thread.

Solution:

• Only query for library roots when viewing a source file that does an import
• Show warning and provide intention action when package sources do not exist (but dependency exists)