perl/Makefile: move "mkdir -p" to module installation loop for maintainability

perl/Makefile: move "mkdir -p" to module installation loop for maintainability

In the NO_PERL_MAKEMAKER=YesPlease fallback case, make the directory
that will contain each module when installing it (simulating "install
-D") instead of hardcoding "Git/SVN/Memoize is the deepest level".
This should make this codepath which is not used often on development
machines a little easier to maintain.

Requested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>