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/microdroid/initrd/Android.bp b/microdroid/initrd/Android.bp
index a9d0da3..d8e7069 100644
--- a/microdroid/initrd/Android.bp
+++ b/microdroid/initrd/Android.bp
@@ -7,7 +7,7 @@
     srcs: ["src/main.rs"],
     rustlibs: [
         "libanyhow",
-        "libstructopt",
+        "libclap",
     ],
     prefer_rlib: true,
 }