Rework a whole bunch of build stuff

[DHowett-MSFT]

This draft pull request contains a number of interesting changes. Let's talk about the results first.

• All dependencies can be removed from WindowsTerminal except TerminalApp
• All normal-build xcopy rules have been removed
• All project build output (including the MSIX) goes in /obj and /bin.
• Almost all build configuration between cppwinrt and non-cppwinrt projects has been unified (cppwinrt.build.pre.props vs common.build.pre.props differs with regards to warnings #1155)
• TerminalApp no longer needs to manually include TerminalAppLib's stuff
  • It can use a normal ProjectReference
• The package works.
• CascadiaPackage no longer needs to include nuget packages itself
• The application is still double-click activatable
• PRI files are automatically rolled up
• The package is 17MB because it includes TerminalAppLib.lib (👎)

Now, the how:

• WindowsTerminal now defines GetPackagingOutputs, which recurses into its children (just like an appx project)
• cppwinrt.build.pre.props now defers to common.build.pre.props for most of its configuration (and loses a bunch of redundant configuration)
• OutDir is set before including the C++ machinery (this turned out to be important (!))
• CascadiaPackage has been given a nudge towards including all subproject packaging outputs

This will still not work properly on Visual Studio 16.3, pending a fix for TerminalAppLib/TerminalApp that @zadjii-msft is cooking up.

[zadjii-msft]

Wait, that's it? That seems way easier than I thought it should be.

I recall there was problems with mdmergeing when cppwinrt projects would binplace to bin/. Does that work now? For both clean builds and partial rebuilds?

[zadjii-msft]

@DHowett-MSFT Do we still want to do this, Or should we abandon it? If 0.3 goes out cleanly, I guess that gives us another window to mess with the build, but I'd also love to not have it broken for weeks again.

• What benefits does this get us, other than slightly cleaner project files? Does it fix something or enable something?
• Will the release appx still work from the build pipeline?

[DHowett-MSFT]

We still want this, but as part of a more holistic change that fixes #922.