Use fileKey instead of canonicalPath when deduping files

`resolveDependencies` showed up as a hotspot when profiling due to `canonicalPath` doing a lot of file system calls.
`fileKey` is more efficient and should guarantee uniqueness (`canonicalPath` may fail in the case of hardlinks).
The new code is 3x faster in our use case.

compiler/util-io/src/org/jetbrains/kotlin/konan/file/File.kt

@@ -56,6 +56,18 @@ data class File constructor(internal val javaPath: Path) {
     val listFilesOrEmpty: List<File>
         get() = if (exists) listFiles else emptyList()
 
+	// A fileKey is an object that uniquely identifies the given file.
+	val fileKey: Any
+	    get() {
+            // It is not guaranteed that all filesystems have fileKey. If not we fall
+            // back on canonicalPath which can be significantly slower to get.
+            var key = Files.readAttributes(javaPath, BasicFileAttributes::class.java).fileKey()
+            if (key == null) {
+                key = this.canonicalPath
+            }
+            return key
+        }
+
     fun child(name: String) = File(this, name)
     fun startsWith(another: File) = javaPath.startsWith(another.javaPath)