Update swift_library logic for module name generation

* Modules are now named after its full target path by default.
* module_name attribute can be used to override this logic.

--
MOS_MIGRATED_REVID=123422704
diff --git a/examples/swift/foo.swift b/examples/swift/foo.swift
index 067c673..1dd35c4 100644
--- a/examples/swift/foo.swift
+++ b/examples/swift/foo.swift
@@ -1,4 +1,4 @@
-import class BarLib.Multiplier
+import class examples_BarLib.Multiplier
 
 public class Foo {
   public init() {}