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/SchemaGenerator.cs

@@ -0,0 +1,98 @@
+/*************************************************************************
+ * ModernUO                                                              *
+ * Copyright 2019-2021 - ModernUO Development Team                       *
+ * Email: hi@modernuo.com                                                *
+ * File: SchemaGenerator.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.Collections.Immutable;
+using System.IO;
+using System.Linq;
+using System.Text.Json;
+using Microsoft.CodeAnalysis;
+using SerializableMigration;
+using SourceGeneration;
+
+namespace SerializationSchemaGenerator
+{
+    public static class SchemaGenerator
+    {
+        public static void GenerateSchema(
+            this Compilation compilation,
+            INamedTypeSymbol classSymbol,
+            AttributeData serializableAttr,
+            ImmutableArray<ISymbol> fieldsAndProperties,
+            string migrationPath,
+            JsonSerializerOptions jsonSerializerOptions,
+            ImmutableArray<INamedTypeSymbol> serializableTypes
+        )
+        {
+            var serializableFieldAttribute =
+                compilation.GetTypeByMetadataName(SymbolMetadata.SERIALIZABLE_FIELD_ATTRIBUTE);
+
+            var version = (int)serializableAttr.ConstructorArguments[0].Value!;
+
+            var serializablePropertySet = new SortedSet<SerializableProperty>(new SerializablePropertyComparer());
+
+            foreach (var fieldOrPropertySymbol in fieldsAndProperties)
+            {
+                var allAttributes = fieldOrPropertySymbol.GetAttributes();
+
+                var serializableFieldAttr = allAttributes
+                    .FirstOrDefault(
+                        attr =>
+                            SymbolEqualityComparer.Default.Equals(attr.AttributeClass, serializableFieldAttribute)
+                    );
+
+                if (serializableFieldAttr == null)
+                {
+                    continue;
+                }
+
+                var order = (int)serializableFieldAttr.ConstructorArguments[0].Value!;
+
+                var serializableProperty = SerializableMigrationRulesEngine.GenerateSerializableProperty(
+                    compilation,
+                    fieldOrPropertySymbol,
+                    order,
+                    allAttributes,
+                    serializableTypes
+                );
+
+                if (serializableProperty == null)
+                {
+                    continue;
+                }
+
+                serializablePropertySet.Add(serializableProperty);
+            }
+
+            var serializableProperties = serializablePropertySet.ToImmutableArray();
+
+            // Write the migration file
+            var newMigration = new SerializableMetadata
+            {
+                Version = version,
+                Type = classSymbol.ToDisplayString(),
+                Properties = serializableProperties
+            };
+            WriteMigration(migrationPath, newMigration, jsonSerializerOptions);
+        }
+
+        public static void WriteMigration(string migrationPath, SerializableMetadata metadata, JsonSerializerOptions options)
+        {
+            Directory.CreateDirectory(migrationPath);
+            var filePath = Path.Combine(migrationPath, $"{metadata.Type}.v{metadata.Version}.json");
+            File.WriteAllText(filePath, JsonSerializer.Serialize(metadata, options));
+        }
+    }
+}