The global --topdir-format make:XXX option to jw-pkg is half-baked at
best, and __find_dir() ignores it entirely. Make __find_dir() return
some Makefile-syntax-formatted output if the option is present. Not
used anywhere, currently, and, hence, badly tested, but still better
than the situation before.
Signed-off-by: Jan Lindemann <jan@janware.com>