[avb] Move PartitionName and util methods to separate modules

This is a refactoring. There's no behavior change in this cl.

Bug: 256148034
Test: m pvmfw_img && atest libpvmfw_avb.integration_test
Change-Id: I3926076293cead03db0ca3409fc82fc120cb80ce
diff --git a/pvmfw/avb/src/lib.rs b/pvmfw/avb/src/lib.rs
index 6a5b16d..8142674 100644
--- a/pvmfw/avb/src/lib.rs
+++ b/pvmfw/avb/src/lib.rs
@@ -19,6 +19,8 @@
 #![feature(mixed_integer_ops)]
 
 mod error;
+mod partition;
+mod utils;
 mod verify;
 
 pub use error::AvbSlotVerifyError;