move generator/ to src/

A long time ago there was the idea to use src/ for something else,
but that is no longer relevant. Its nice to switch to a more
conventional repo layout.

src/index.zig

@@ -0,0 +1,8 @@
+pub const generateVk = @import("vulkan/generator.zig").generate;
+pub const VkGenerateStep = @import("vulkan/build_integration.zig").GenerateStep;
+pub const ShaderCompileStep = @import("build_integration.zig").ShaderCompileStep;
+
+test "main" {
+    _ = @import("xml.zig");
+    _ = @import("vulkan/c_parse.zig");
+}