Move loopdevice module to dm_rust lib

This is a generic module, not specific to apkdmverity. Will be reused
for unit-testing other dm- devices. Also, setup the test infra for
dm_rust module...

Bug: 250880499
Test: atest libdm_rust.test
Test: atest apkdmverity.test
Change-Id: I32971938908ea8c0213583885403910baac8be10
diff --git a/libs/devicemapper/src/lib.rs b/libs/devicemapper/src/lib.rs
index 1bcaf1a..938ca0f 100644
--- a/libs/devicemapper/src/lib.rs
+++ b/libs/devicemapper/src/lib.rs
@@ -42,6 +42,8 @@
 pub mod util;
 /// Exposes the DmVerityTarget & related builder
 pub mod verity;
+// Expose loopdevice
+pub mod loopdevice;
 
 mod sys;
 use sys::*;