Support ::mylib which that target that exists at configure-time

[mrexodia]

[project]
name = "namespace"

[target.mylib]
type = "static"
sources = ["src/mylib.cpp"]

[target.myexe]
type = "executable"
sources = ["src/myexe.cpp"]
link-libraries = ["::mylibb"]

This will emit the following CMake:

if(NOT TARGET mylibb)
	message(FATAL_ERROR "Target \"mylibb\" referenced by \"myexe\" does not exist!")
endif()

target_link_libraries(
	mylibb
)

At configure-time it will error:

CMake Error at CMakeLists.txt:52 (message):
  Target "mylibb" referenced by "myexe" does not exist!

Relevant CMake issue: https://gitlab.kitware.com/cmake/cmake/-/issues/26497