[Cherry pick][WinAppSDK 1.5] Only merge PRIs from references if the project is producing an executable

[evelynwu-msft]

(Addresses microsoft/microsoft-ui-xaml#8857)

The MRT Core targets are incorrectly merging the PRIs of references into the final PRI in all cases. Suppose you have Package A which contains some WinUI controls, which results in some XBFs being inserted as resources into its PRI. There is also a Package B that relies on Package A version 1.0. Finally, there is an App C that relies on both Package A (version 1.1) and Package B. Because Package B is including the PRI resources from Package A version 1.0 in its own PRI, when App C attempts to merge the PRI resources from all of its dependencies a conflict will arise and break the build.

Package B should not be including the PRI resources from Package A in its own PRI, and instead should advertise the dependency and allow App C to resolve it and perform the PRI resource merge itself. In the UWP targets, this was done by checking to see if the project was producing a package but since Windows App SDK needs to also handle unpackaged apps, the correct way to achieve the same goal is to check if the project is a library (i.e. not producing an executable) and not merge PRI resources from dependencies if that is the case.

Note: Libraries (including Nuget packages) affected by this bug will need to be recompiled.

Cherry-picked from #4124

[evelynwu-msft]

/azp run

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[codendone]