Minor. Get rid of static public parts in MainFunctionDetector

In further commits, MainFunctionDetector::isMain will use
getFunctionDescriptor property

idea/src/org/jetbrains/kotlin/idea/inspections/MainFunctionReturnUnitInspection.kt

@@ -15,6 +15,7 @@ import org.jetbrains.kotlin.builtins.KotlinBuiltIns
 import org.jetbrains.kotlin.descriptors.FunctionDescriptor
 import org.jetbrains.kotlin.idea.MainFunctionDetector
 import org.jetbrains.kotlin.idea.caches.resolve.analyze
+import org.jetbrains.kotlin.idea.caches.resolve.resolveToDescriptorIfAny
 import org.jetbrains.kotlin.idea.core.setType
 import org.jetbrains.kotlin.idea.search.usagesSearch.descriptor
 import org.jetbrains.kotlin.name.FqName
@@ -35,7 +36,7 @@ class MainFunctionReturnUnitInspection : AbstractKotlinInspection() {
 
             val descriptor = function.descriptor as? FunctionDescriptor ?: return
             if (isMain) {
-                if (!MainFunctionDetector.isMain(descriptor, checkReturnType = false)) return
+                if (!MainFunctionDetector { it.resolveToDescriptorIfAny() }.isMain(descriptor, checkReturnType = false)) return
             } else {
                 val junitTestFqNames = listOf(FqName("org.junit.Test"), FqName("org.junit.jupiter.api.Test"))
                 if (testAnnotations.none { it.fqName() in junitTestFqNames }) return
@@ -71,4 +72,4 @@ private class ChangeMainFunctionReturnTypeToUnitFix(private val hasExplicitRetur
             function.setType(KotlinBuiltIns.FQ_NAMES.unit.asString())
         }
     }
-}
+}

idea/src/org/jetbrains/kotlin/idea/inspections/UnusedReceiverParameterInspection.kt

@@ -77,7 +77,7 @@ class UnusedReceiverParameterInspection : AbstractKotlinInspection() {
 
                 val callable = callableDeclaration.descriptor ?: return
 
-                if (MainFunctionDetector.isMain(callable)) return
+                if (MainFunctionDetector { it.resolveToDescriptorIfAny() }.isMain(callable)) return
 
                 val containingDeclaration = callable.containingDeclaration
                 if (containingDeclaration != null && containingDeclaration == receiverTypeDeclaration) {
@@ -175,4 +175,4 @@ fun isUsageOfDescriptor(descriptor: DeclarationDescriptor, resolvedCall: Resolve
     }
 
     return false
-}
+}