KTIJ-26713 [AA] Clean-up KtFirReferenceShortener

- revert some accidental changes in
`findSmallestElementOfTypeContainingSelection` function (see 48433bf9)
- simplify `dropFakeRootPrefixIfPresent` by using `tail`, add a new
test-case to check that it works
- simplify `findClassifierElementsToShorten` by not passing lambdas and
calling a common functions instead

analysis/analysis-api-standalone/tests-gen/org/jetbrains/kotlin/analysis/api/standalone/fir/test/cases/generated/cases/references/FirStandaloneNormalAnalysisSourceModuleReferenceShortenerTestGenerated.java

@@ -292,6 +292,12 @@ public class FirStandaloneNormalAnalysisSourceModuleReferenceShortenerTestGenera
         runTest("analysis/analysis-api/testData/components/referenceShortener/referenceShortener/kdocQualifierSelected.kt");
     }
 
+    @Test
+    @TestMetadata("kdocQualifierSelected_rootIdePrefix.kt")
+    public void testKdocQualifierSelected_rootIdePrefix() throws Exception {
+        runTest("analysis/analysis-api/testData/components/referenceShortener/referenceShortener/kdocQualifierSelected_rootIdePrefix.kt");
+    }
+
     @Test
     @TestMetadata("kdocUnresolved.kt")
     public void testKdocUnresolved() throws Exception {