Update building instructions (#391)

Building.md

@@ -7,35 +7,28 @@ This project is configured to build with both CMake and the Swift Package Manage
 
 Pull requests are validated with a GitHub Actions workflow which builds using both build systems and runs tests from the SPM build.
 
-## CMake
-
-To build with CMake, open a `VS Developer Command Prompt` at the root of this repo and type:
-
-```cmd
-cmake --preset debug
-cmake --build --preset debug
-```
-
-When building from the root of the repo, the Generator project will be built during the configure phase, so that the resulting executable can be used to generate test projections later in the build.
-
-The repo is configured such that many subdirectories can be built the same way, to scope down the build.
-
-You can also use [Visual Studio Code](https://code.visualstudio.com/) with the [CMake Tools extension](https://marketplace.visualstudio.com/items?itemName=ms-vscode.cmake-tools) to configure and build, as long as you start it from within a `VS Developer Command Prompt`.
-
-## SPM
-
-The root of the repo is an SPM package that builds the support module and can run its tests. This allows SPM references to the git repo to resolve to the support module.
-
-```cmd
-swift build --build-tests
-swift test --skip-build
-```
-
-The `/Generator` subdirectory is an independent SPM package that builds the code generator and can run its tests:
-
-```cmd
-Generator> swift build --build-tests
-Generator> swift test --skip-build
-```
-
-The `/InteropTests` subdirectory requires building `WinRTComponent.winmd` and `WinRTComponent.dll`, and invoking `SwiftWinRT.exe`, which SPM cannot do. A helper script, `SPMPrebuild.ps1`, will do that using CMake, after which the package can be built and tested normally.
+## Prerequisites
+
+- Windows 10 or above (building on other platforms should be possible but is untested)
+- A Swift toolchain, version 5.10 or above
+  - `winget install --id Swift.Toolchain`
+- Visual Studio (any edition) with the Desktop C++ Workload
+  - `winget install --id Microsoft.VisualStudio.2022.Community --override "--quiet --add Microsoft.VisualStudio.Workload.NativeDesktop"`
+- A Windows SDK (any reasonably recent version)
+  - Comes with Visual Studio, or use `winget install --id Microsoft.WindowsSDK.10.0.22000`
+- CMake and ninja
+  - Comes with Visual Studio, or use `winget install --id Kitware.CMake` and `winget install --id Ninja-build.Ninja`
+- NuGet
+  - `winget install Microsoft.NuGet`
+
+## In Visual Studio Code
+Open Visual Studio Code from a `x64 Native Tools Command Prompt for VS 2022` and use standard IDE commands for building (<kbd>ctrl</kbd>+<kbd>shift</kbd>+<kbd>B</kbd>). Subfolders `Generator` and `InteropTests` can also be opened for a scoped down development environment.
+
+## From the command line
+1. Open a `x64 Native Tools Command Prompt for VS 2022`.
+2. Refer to the [Build and Test workflow](.github/workflows/build-and-test.yml) for build commands.
+  - With CMake, the Generator project will be built during the configure phase so that the resulting executable can be used to generate test projections later on, but the tests themselves will not be built due to XCTest dependencies on SPM.
+  - With SPM, the build is split in several steps:
+    - The root `Package.swift`, which defines modules supporting the generated code.
+    - The `Generator` subfolder, which builds the code genreator.
+    - The `InteropTests` subfolder, which builds tests against a WinRT component and relies on a prebuild Powershell script.