ComputeImports: fix package computation.

[mbien]

attempt to fix #7073, #5537 and #6902

• getPackageElement() requires the full package String or it returns null (e.g javax is not sufficient)
• this caused a bug where the fix-imports-action imported members which matched root packages of random fully qualified declarations (e.g imports)

 // hit ctrl+shift+i, it shouldn't import anything
import java.util.List;
public class FixImports {
    public static class SomeClass2 {
        public static String java(String value) {
            return value;
        }
    }
}

[mbien]

Minimal nitpick: The test sources seem to be missing package declarations.

@matthiasblaesing good that you mentioned this since I misunderstood how the test works. It uses now one test file since all it has to do is to check that the computed candidates are empty.