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/testData/components/referenceShortener/referenceShortener/kdocQualifierSelected_rootIdePrefix.kt

@@ -0,0 +1,14 @@
+// FILE: main.kt
+import dependency.Foo
+
+/**
+ * [<expr>_root_ide_package_.dependency.Foo</expr>.foo]
+ */
+fun test() {}
+
+// FILE: dependency.kt
+package dependency
+
+object Foo {
+    fun foo() {}
+}

analysis/analysis-api/testData/components/referenceShortener/referenceShortener/kdocQualifierSelected_rootIdePrefix.txt

@@ -0,0 +1,9 @@
+Before shortening: _root_ide_package_.dependency.Foo
+with DO_NOT_SHORTEN:
+[kdoc] _root_ide_package_.dependency
+with SHORTEN_IF_ALREADY_IMPORTED:
+[kdoc] _root_ide_package_.dependency.Foo
+with SHORTEN_AND_IMPORT:
+[kdoc] _root_ide_package_.dependency.Foo
+with SHORTEN_AND_STAR_IMPORT:
+[kdoc] _root_ide_package_.dependency.Foo