install: add root directory to LookupPaths structure

install: add root directory to LookupPaths structure

We use the root directory parameter while putting together the LookupPaths
structure, hence let's also store it in the structure as-is. That way we can
drop a parameter from half of the functions in install.c

Also, let's move the validation of the root paths into lookup_paths_init() so
that we can drop even more code from install.c