GH-172: Fix dependency conflict resolution

This fixes the outstanding issue at GH-172 that creates an
issue when mixing project and plugin-based dependencies that
could be on different versions transitively. This only now
works because of GH-215 fixing the way dependencies get
resolved... we now resolve both project and plugin
dependencies at the exact same time, which allows for some
version resolution and conflict fixes to be performed
by Eclipse Aether.

protobuf-maven-plugin/src/main/java/io/github/ascopes/protobufmavenplugin/generation/SourceCodeGenerator.java

@@ -85,8 +85,8 @@ public boolean generate(GenerationRequest request) throws ResolutionException, I
     final var protocPath = discoverProtocPath(request);
 
     final var resolvedPlugins = discoverPlugins(request);
-    final var importPaths = discoverImportPaths(request);
     final var sourcePaths = discoverCompilableSources(request);
+    final var importPaths = discoverImportPaths(sourcePaths, request);
 
     if (sourcePaths.isEmpty()) {
       if (request.isFailOnMissingSources()) {
@@ -173,17 +173,24 @@ private Collection<ResolvedProtocPlugin> discoverPlugins(
   }
 
   private Collection<ProtoFileListing> discoverImportPaths(
+      Collection<ProtoFileListing> sourcePathListings,
       GenerationRequest request
   ) throws IOException, ResolutionException {
-    var artifacts = concat(request.getImportDependencies(), request.getSourceDependencies());
     var artifactPaths = artifactPathResolver.resolveDependencies(
-        artifacts,
+        request.getImportDependencies(),
         request.getDependencyResolutionDepth(),
         !request.isIgnoreProjectDependencies()
     );
-    var importPaths = request.getImportPaths();
 
-    return protoListingResolver.createProtoFileListings(concat(artifactPaths, importPaths));
+    var importPathListings = protoListingResolver
+        .createProtoFileListings(concat(request.getImportPaths(), artifactPaths));
+
+    // Use the source paths here as well and use them first to give them precedence. This works
+    // around GH-172 where we can end up with different versions on the import and source paths
+    // depending on how dependency conflicts arise.
+    return Stream.concat(sourcePathListings.stream(), importPathListings.stream())
+        .distinct()
+        .collect(Collectors.toList());
   }
 
   private Collection<ProtoFileListing> discoverCompilableSources(