Improve the dependency creation example to support subdirectories

[hiker]

ATM the output of the dependency creation tool does not support the output of subdirectories, e.g.:

$ cd fparser/example
$ ./create_dependencies.py test_files/*
$ cd test_files/
$ ../create_dependencies.py *
b.o: a.o
c.o: a.o b.o

In the first case the following output would be useful:

$ ./create_dependencies.py test_files/*
test_files/b.o: test_files/a.o
test_files/c.o: test_files/a.o test_files/b.o

since this better supports files in different directories, and out-of-source-tree builds which maintains the directory structure (like LFRic does, and the PSyclone infrastructure will support).