WiX: simplify the runtime package definition (#206)

- Use durable GUID generation for auto-generation of the GUID.
- Combine the platform definitions into a singular source to avoid drift.

This avoids having to keep the variants in sync and provides an easier
upgrade path. This is the first step towards enabling a non-admin
experience for users.

Author: compnerd

platforms/Windows/devtools.wixproj

@@ -19,7 +19,7 @@
   <Import Project="WiXCodeSigning.targets" />
 
   <PropertyGroup>
-    <DefineConstants>ProductVersion=$(ProductVersion);DEVTOOLS_ROOT=$(DEVTOOLS_ROOT);</DefineConstants>
+    <DefineConstants>ProductArchitecture=$(ProductArchitecture);ProductVersion=$(ProductVersion);DEVTOOLS_ROOT=$(DEVTOOLS_ROOT);</DefineConstants>
     <HarvestDirectoryAutogenerateGuids>false</HarvestDirectoryAutogenerateGuids>
     <HarvestDirectoryGenerateGuidsNow>true</HarvestDirectoryGenerateGuidsNow>
     <HarvestDirectoryNoLogo>true</HarvestDirectoryNoLogo>