Use "wixext4" instead of "tools" as the root folder in ".wixext.nupkg"s

[robmen]

• Describe the scenario and benefits that the feature supports.

The tools folder in .nupkgs is for tools. WiX extensions are not tools and not reference libraries so we should use a unique folder.

• Describe how you're accomplishing the feature today (if possible).

Today, the tools folder is being used as a catch-all for non-reference assemblies.

• Describe what you'd like the new feature to do.

Update WiX v4 extensions to use a wixext4 folder as the root of wixext.nupkg. Adding a number to the root folder allows future WiX extensions to support multiple versions of the WiX toolset in a single .wixext.nupkg (for example, wixext4 and wixext5).