Enforce correct rust library file names.

rustc expects libraries and proc_macro filenames to conform to
a particular format, alphanumeric with underscores and lib${crate_name}.*.
Enforce this with a check when getStem() is called.

This makes the crate_name property required for proc_macros and
libraries. This also removes the notion of a default crate name derived
from the module name. It's not needed for binaries, so this won't impact
them.

Bug: 143579265
Test: m -j crosvm.experimental
Change-Id: I2770cf7d02dd4291c3d240d58d242b940098dcee

rust/builder.go

@@ -88,7 +88,9 @@ func transformSrctoCrate(ctx android.ModuleContext, main android.Path,
 	rustcFlags = append(rustcFlags, flags.GlobalRustFlags...)
 	rustcFlags = append(rustcFlags, flags.RustFlags...)
 	rustcFlags = append(rustcFlags, "--crate-type="+crate_type)
-	rustcFlags = append(rustcFlags, "--crate-name="+crate_name)
+	if crate_name != "" {
+		rustcFlags = append(rustcFlags, "--crate-name="+crate_name)
+	}
 	if targetTriple != "" {
 		rustcFlags = append(rustcFlags, "--target="+targetTriple)
 		linkFlags = append(linkFlags, "-target "+targetTriple)