rust: Add support for multiple protos per module.

This further emulates the rust-protobuf gen_mod_rs flag by providing
support for generating a single module containing multiple protobuf
definitions.

Bug: 171361369
Test: New Soong tests.
Test: Example module containing multiple protos works.
Change-Id: I815f9628a8289ae512758073dac49bc4535abf01
diff --git a/rust/source_provider.go b/rust/source_provider.go
index 436518c..7719611 100644
--- a/rust/source_provider.go
+++ b/rust/source_provider.go
@@ -30,6 +30,7 @@
 type BaseSourceProvider struct {
 	Properties SourceProviderProperties
 
+	// The first file in OutputFiles must be the library entry point.
 	OutputFiles      android.Paths
 	subAndroidMkOnce map[SubAndroidMkProvider]bool
 	subName          string