[FIR] KT-56505 Fix checkVisibilityModifier taking wrong modifier list

- KT-56505 occurred because `source.getChild(KtNodeTypes.MODIFIER_LIST)`
  returns any modifier list in the subtree of the source element, not
  necessarily the modifier list belonging to the checked element.
  `depth = 1` restricts the search to the modifier list belonging to the
  checked element itself.
- For example, given `f1` from KT-56505, `getChild` would return the
  modifier list of `public var foo = 0`. Because it contains a
  visibility modifier, `f1` wasn't marked with
  `NO_EXPLICIT_VISIBILITY_IN_API_MODE`.

^KT-56505 fixed

compiler/fir/analysis-tests/testData/resolve/extendedCheckers/RedundantVisibilityModifierCheckerWithExplicitApiMode.kt

@@ -1,6 +1,6 @@
 // EXPLICIT_API_MODE: STRICT
 
-fun f1() {
+<!NO_EXPLICIT_VISIBILITY_IN_API_MODE!>fun f1<!>() {
     class LocalClass {
         public var foo = 0
     }
@@ -71,7 +71,7 @@ public interface I2 {
 
 public var baz2: Int = 0
 
-class J1 {
+<!NO_EXPLICIT_VISIBILITY_IN_API_MODE!>class J1<!> {
     protected val baz: Int = 0
         <!REDUNDANT_VISIBILITY_MODIFIER!>protected<!> get() = field * 2
     <!NO_EXPLICIT_VISIBILITY_IN_API_MODE!>var baf<!>: Int = 0
@@ -80,7 +80,7 @@ class J1 {
             field = value
         }
 
-    var buf: Int = 0
+    <!NO_EXPLICIT_VISIBILITY_IN_API_MODE!>var buf<!>: Int = 0
         <!GETTER_VISIBILITY_DIFFERS_FROM_PROPERTY_VISIBILITY!>private<!> get() = 42
         protected set(value) {
             field = value