fix(codegen): Creates multiple steps for serialization source generator (#628)

Roslyn Source generators are not supposed to access I/O. To get around this we have to use `AdditionalFiles` to give the analyzer access to read/load the schema files.
To write the schema files we have to use a separate program altogether.

- [X] Adds SerializationSchemaGenerator
- [X] Splits out the SerializationGenerator code

Projects/SerializationSchemaGenerator/Application.cs

@@ -0,0 +1,84 @@
+/*************************************************************************
+ * ModernUO                                                              *
+ * Copyright 2019-2021 - ModernUO Development Team                       *
+ * Email: hi@modernuo.com                                                *
+ * File: Application.cs                                                  *
+ *                                                                       *
+ * This program is free software: you can redistribute it and/or modify  *
+ * it under the terms of the GNU General Public License as published by  *
+ * the Free Software Foundation, either version 3 of the License, or     *
+ * (at your option) any later version.                                   *
+ *                                                                       *
+ * You should have received a copy of the GNU General Public License     *
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>. *
+ *************************************************************************/
+
+using System;
+using System.Collections.Immutable;
+using System.IO;
+using System.Text.Json;
+using System.Threading.Tasks;
+using SerializationGenerator;
+
+namespace SerializationSchemaGenerator
+{
+    public static class Application
+    {
+        public static void Main(string[] args)
+        {
+            if (args.Length < 1)
+            {
+                throw new ArgumentException("Usage: dotnet SerializationSchemaGenerator.dll <path to solution>");
+            }
+
+            var solutionPath = args[0];
+
+            Parallel.ForEach(
+                SourceCodeAnalysis.GetCompilation(solutionPath),
+                (projectCompilation) =>
+                {
+                    var (project, compilation) = projectCompilation;
+                    if (project.Name.EndsWith(".Tests", StringComparison.Ordinal) || project.Name == "Benchmarks")
+                    {
+                        return;
+                    }
+
+                    var projectFile = new FileInfo(project.FilePath!);
+                    var migrationPath = Path.Join(projectFile.Directory?.FullName, "Migrations");
+                    Directory.CreateDirectory(migrationPath);
+
+                    var syntaxReceiver = new SerializerSyntaxReceiver();
+
+                    foreach (var syntaxTree in compilation.SyntaxTrees)
+                    {
+                        var root = syntaxTree.GetRoot();
+                        var syntaxVisitor = new SyntaxVisitor(compilation.GetSemanticModel(syntaxTree), syntaxReceiver);
+                        syntaxVisitor.Visit(root);
+                    }
+
+                    var jsonOptions = new JsonSerializerOptions
+                    {
+                        WriteIndented = true,
+                        AllowTrailingCommas = true,
+                        IgnoreNullValues = true,
+                        ReadCommentHandling = JsonCommentHandling.Skip
+                    };
+
+                    var serializableTypes = syntaxReceiver.SerializableList;
+
+                    foreach (var (classSymbol, (attributeData, fieldsList)) in syntaxReceiver.ClassAndFields)
+                    {
+                        compilation.GenerateSchema(
+                            classSymbol,
+                            attributeData,
+                            fieldsList.ToImmutableArray(),
+                            migrationPath,
+                            jsonOptions,
+                            serializableTypes
+                        );
+                    }
+                }
+            );
+        }
+    }
+}