Move Path extensions to the new package kotlin.io.path

This avoids the conflict with the existing top-level functions in
kotlin.io package.

#KT-19192

libraries/stdlib/jdk7/test/PathExtensionsTest.kt

@@ -7,6 +7,7 @@ package kotlin.jdk7.test
 
 import java.io.IOException
 import java.nio.file.*
+import kotlin.io.path.*
 import kotlin.test.*
 
 class PathExtensionsTest {