Allow imported INTERFACE libraries to specify a link library name

Add an `IMPORTED_LIBNAME[_<CONFIG>]` target property to specify a library
name to be placed on the link line in place of an interface library
since it has no library file of its own.  Restrict use of the property
to imported `INTERFACE` libraries.

This will be particularly useful for find modules that need to provide
imported libraries from system SDKs where the full path to the library
file is not known.  Now such find modules will be able to provide an
imported interface library and set `IMPORTED_LIBNAME` to refer to the
SDK library by name.

Issue: #15267

Tests/InterfaceLibrary/item.cpp

@@ -0,0 +1,4 @@
+int item()
+{
+  return 0;
+}