Use the CppWinRTPlatformWinMDReferences itemgroup as the input for the platform projection.

[jlaanstra]

Fixes #773

TODO

• Validate razzle build.

[jlaanstra]

Validated the razzle build after reverting #742

[DHowett]

terminal is seeing some build failures due to platform types missing when transitively used in projects that depend on winmds produced by other components with the latest version; could this be related?

MDMERGE : error MDM2002: The type "Windows.Foundation.IReference`1" defined in file Microsoft.Terminal.TerminalControl.winmd was referenced by type Microsoft.Terminal.TerminalControl.ICoreSettings but could not be found. [src\cascadia\UnitTests_TerminalCore\UnitTests.vcxproj]

cppwinrt : error Type 'Windows.Foundation.EventRegistrationToken' could not be found
 method: TerminalOutput
 type: Microsoft.Terminal.TerminalConnection.ITerminalConnection
 database: C:\Users\Dustin\src\terminal\x64\Release\TerminalConnection\Microsoft.Terminal.TerminalConnection.w
inmd

[DHowett]

(Rolling back one version to 2.0.201017.1 fixes our build. 2.0.201026 is broken; this seems to be the only changeset that addresses platform winmds in the new version.)

[jlaanstra]

@DHowett can you share a binlog from your build? Happy to take a look. You can probably apply the same fix from #781 to get unblocked as your unit test project is probably not targeting UAP and so the default targets don't add the WinMDs.