Allow use of local targets instead of imported ones

To support the LHCb satellite projects use case, I added the option GAUDI_PREFER_LOCAL_TARGETS. When it's set to true, the arguments to the LINK option of gaudi_add_* commands will drop the namespace part of the target if a local target with the same name exist.

So if an already processed subdirectory defines the library MyStuff, the following code

set(GAUDI_PREFER_LOCAL_TARGETS TRUE)
gaudi_add_library(MyOtherStuff SOURCES src/abc.cpp
    LINK PUBLIC SomeProject::MyStuff)

will be equivalent to

set(GAUDI_PREFER_LOCAL_TARGETS TRUE)
gaudi_add_library(MyOtherStuff SOURCES src/abc.cpp
    LINK PUBLIC MyStuff)

The option GAUDI_PREFER_LOCAL_TARGETS is OFF by default.