install,export: Maybe transform OBJECT libraries to INTERFACE libraries
Teach the `install` and `export` commands to support installing and exporting `OBJECT` libraries without their object files. Transform them to `INTERFACE` libraries in such cases. For `install(TARGETS)`, activate this when no destination for the object files is specified. For `export`, activate this only under Xcode with multiple architectures when we have no well-defined object file locations to give to clients.
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