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/SerializationGenerator/SerializableMigration/SerializableMigrationSchema.cs

@@ -0,0 +1,78 @@
+/*************************************************************************
+ * ModernUO                                                              *
+ * Copyright 2019-2021 - ModernUO Development Team                       *
+ * Email: hi@modernuo.com                                                *
+ * File: SerializableMigrationSchema.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.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Text.Json;
+using System.Text.RegularExpressions;
+using Microsoft.CodeAnalysis;
+
+namespace SerializableMigration
+{
+    public static class SerializableMigrationSchema
+    {
+        public static JsonSerializerOptions GetJsonSerializerOptions() =>
+            new()
+            {
+                WriteIndented = true,
+                AllowTrailingCommas = true,
+                IgnoreNullValues = true,
+                ReadCommentHandling = JsonCommentHandling.Skip
+            };
+
+        private static Dictionary<string, SerializableMetadata> _cache = new();
+
+        private static Regex _fileRegex = new(@"\S+\.v\d+\.json$");
+
+        public static List<SerializableMetadata> GetMigrationsByAnalyzerConfig(
+            this GeneratorExecutionContext context,
+            INamedTypeSymbol typeSymbol,
+            int version,
+            JsonSerializerOptions options
+        )
+        {
+            var typeName = typeSymbol.ToDisplayString();
+            var migrations = new SortedSet<SerializableMetadata>(new SerializableMetadataComparer());
+
+            foreach (var additionalText in context.AdditionalFiles)
+            {
+                var fi = new FileInfo(additionalText.Path);
+                if (!_fileRegex.IsMatch(fi.Name))
+                {
+                    continue;
+                }
+
+                if (!_cache.TryGetValue(fi.Name, out var migration))
+                {
+                    var text = additionalText.GetText(context.CancellationToken)?.ToString();
+                    if (text == null)
+                    {
+                        continue;
+                    }
+
+                    migration = JsonSerializer.Deserialize<SerializableMetadata>(text, options);
+                }
+
+                if (typeName == migration!.Type && version > migration.Version)
+                {
+                    migrations.Add(migration);
+                }
+            }
+
+            return migrations.ToList();
+        }
+    }
+}