Exclude public API symbols in explicit mode from unused symbol inspection

#KT-41659 Fixed

idea/testData/inspectionsLocal/unusedSymbol/unusedClassExplicitApi.kt

@@ -0,0 +1,5 @@
+// PROBLEM: none
+// COMPILER_ARGUMENTS: -Xexplicit-api=strict
+
+public class <caret>X {
+}

idea/testData/inspectionsLocal/unusedSymbol/unusedExtensionExplicitApi.kt

@@ -0,0 +1,4 @@
+// PROBLEM: none
+// COMPILER_ARGUMENTS: -Xexplicit-api=strict
+
+public fun String.<caret>baz() {}

idea/testData/inspectionsLocal/unusedSymbol/unusedFunctionExplicitApi.kt

@@ -0,0 +1,6 @@
+// PROBLEM: none
+// COMPILER_ARGUMENTS: -Xexplicit-api=strict
+
+public class X {
+    public fun <caret>foo() {}
+}