Don't try parsing contract-like function not from kotlin package

Returning `null` from `doCheckContract` functions means that we
have failed to parse contract function and should report an error, but
if the called function isn't true contract, we shouldn't evaluate that code
at all.

 #KT-27758 Fixed

compiler/testData/diagnostics/testsWithStdLib/contracts/dsl/callUsualContractFunction.kt

@@ -0,0 +1,42 @@
+// !DIAGNOSTICS: -UNUSED_PARAMETER
+
+package my
+
+// Accepts block, can't be distinguished from kotlin.contract without resolve
+fun contract(block: () -> Unit) {}
+
+// Accepts int, potentially can be distinguished from kotlin.contract by PSI,
+// but as of Kotlin 1.3.0, we don't do that
+fun contract(i: Int) {}
+
+fun doStuff() {}
+
+class SomeClass {
+
+    fun contract() {}
+
+    fun callMemberContractWithThis() {
+        this.contract()
+    }
+
+    fun callMemberContractWithoutThis() {
+        contract()
+    }
+
+    fun callTopLevelSamePsiInMember() {
+        contract { }
+    }
+}
+
+fun callTopLevelSamePsi() {
+    contract { }
+}
+
+fun callTopLevelDifferentPsi() {
+    contract(42)
+}
+
+fun callTopLevelSamePsiNotFirstStatement() {
+    doStuff()
+    contract { }
+}