WIP: Organized obsolescence of Heat in v4

[robmen]

• Describe the scenario and benefits that the feature supports.

heat.exe and its MSBuild tasks (HeatDirectory, HeatFile, HeatProject) are highly desired features but flawed in their design and implementation. In a future release (ideally v5), the concept of harvesting needs to be re-evaluated entirely and better integrated into the WiX toolset. Therefore, the current concept of "Heat" should be supported but clearly on a path to obsolescence.

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

heat.exe consumes resources to maintain in a form factor with no future. That effort would be better spent on pretty much anything else.

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

In WiX v4, wix.exe has emerged as the single executable "to rule them all", which makes heat.exe the "odd .exe out". Given the evolution of the other v3 .exes to commands on wix.exe and its extensions, converting heat.exe to a harvest command would be logical. However, in a future release (ideally the next release, v5), we need to explore the concept of harvesting and how to best integrate it into the build pipeline. At this time, I think it is highly likely language enhancements will be preferred over maintaining a standalone command like wix.exe harvest

To that end, changes to Heat should be focused on extracting it from deep integration within WiX v4 (such as a the direct import of wix.harvesting.targets in wix.targets) while still making heat.exe and the Heat MSBuild tasks available to enable migration from WiX v3 to v4.