Invisible project forgets source paths on classpath update

[rgrunber]

1. Set up the invisible project (include a jar in the lib/ folder)
2. Create a source file that references a class from the jar and either hover over the type or go to the definition to trigger the source file resolution.
3. Update the project classpath by adding a second jar to the lib folder

The type reference should now produce the same result as if it were simply a library with no corresponding sources. The first jar's source classpath should disappear likely due to a bug with the invisible project's classpath updating logic.