Avoid Creating Alias Target If It Already Exists

[threeal]

This issue addresses the lines of code that create an alias target for a specific namespace:

PackageProject.cmake/CMakeLists.txt

Lines 33 to 42 in 2d8a468

	# we want to automatically add :: to our namespace, so only append if a namespace was given in the
	# first place we also provide an alias to ensure that local and installed versions have the same
	# name
	if(DEFINED PROJECT_NAMESPACE)
	if(PROJECT_CPACK)
	set(CPACK_PACKAGE_NAMESPACE ${PROJECT_NAMESPACE})
	endif()
	set(PROJECT_NAMESPACE ${PROJECT_NAMESPACE}::)
	add_library(${PROJECT_NAMESPACE}${PROJECT_NAME} ALIAS ${PROJECT_NAME})
	endif()

It is suggested to create the alias target only if the alias does not already exist; otherwise, it may cause issues as highlighted in boost-ext/ut#564.

Before CMake 3.18, this may not have caused an issue because any new alias target would replace the old alias. However, the behavior has changed since then (see also).