Fix KT-10764 IDEA doesn't show overload conflict between constructor and function...

When checking for overloads in package, consider functions and top-level class constructors as possibly conflicting between each other. NB OverloadUtil uses containing package scope from module descriptor.

Change diagnostic message for CONFLICTING_OVERLOAD: it's misleading in case of fun vs constructor conflict.

Add custom multifile test for diagnostics in IDE (probably not the best; should preprocess file content if it's required to check highlighting in multiple files, not only in the first file).

Add test for KT-10765 Incremental compilation misses overload conflict between constructor and function ...

idea/testData/checker/custom/conflictingOverloadsMultifile1a.kt

@@ -0,0 +1,3 @@
+class A<error>(val x: Int)</error> {
+    <error>constructor()</error>: this(0)
+}

idea/testData/checker/custom/conflictingOverloadsMultifile1b.kt

@@ -0,0 +1,2 @@
+fun A() {}
+fun A(x: Int) = x

idea/testData/checker/custom/conflictingOverloadsMultifile2a.kt

@@ -0,0 +1,2 @@
+<error>fun A()</error> {}
+<error>fun A(x: Int)</error> = x

idea/testData/checker/custom/conflictingOverloadsMultifile2b.kt

@@ -0,0 +1,3 @@
+class A(val x: Int) {
+    constructor(): this(0)
+}