JetExceptionFilter handles correctly the case of several files with the same name

Extract the logic to DebuggerUtils.findSourceFileForClass()

This fixes navigation from stack trace to a file, when there's more than one
file with this name in this package in the project

idea/testData/filters/exceptionFilter/multiSameName/1/foo.kt

@@ -0,0 +1,6 @@
+package multiSameName
+
+fun foo() {
+    val f = { null!! }
+    f()
+}

idea/testData/filters/exceptionFilter/multiSameName/2/foo.kt

@@ -0,0 +1,6 @@
+package multiSameName
+
+fun foo(x: Int) {
+    val f = { null!! }
+    f()
+}