Migrate away from structopt

Clap 3 supports derive natively, so we don't need structopt anymore
(since it was based on clap 2).

Besides, sort the arguments to make positional ones first.

Bug: 246385183
Test: atest AuthFsHostTest ComposHostTestCases MicrodroidTestCase
Change-Id: I75e25ffcb63bf717be8deb9720e0fdf78a0c7ccf
diff --git a/authfs/Android.bp b/authfs/Android.bp
index 7788702..523da35 100644
--- a/authfs/Android.bp
+++ b/authfs/Android.bp
@@ -16,6 +16,7 @@
         "libauthfs_fsverity_metadata",
         "libbinder_rs",
         "libcfg_if",
+        "libclap",
         "libfsverity_digests_proto_rust",
         "libfuse_rust",
         "liblibc",
@@ -24,7 +25,6 @@
         "libopenssl",
         "libprotobuf",
         "librpcbinder_rs",
-        "libstructopt",
         "libthiserror",
     ],
     prefer_rlib: true,