Move path-lookup.c to src/basic

I want to use it from sd-path later on so it needs to be moved out of
src/shared (libsystemd is not allowed to use code from src/shared).

src/basic/meson.build

@@ -159,6 +159,8 @@ basic_sources = files('''
         ordered-set.h
         parse-util.c
         parse-util.h
+        path-lookup.c
+        path-lookup.h
         path-util.c
         path-util.h
         prioq.c