Fix issue 12354: rdmd --makedep(end|file) doesn't output empty rules for all source files

Now rdmd --makedep(end|file) will print an empty rule for each source
file.
rdmd: Improve --makedep(end|file) output format

The current format is not very human friendly and also makes doing
simple checks for testing quite hard. The new format is based on GCC's
format, and it looks like this:

target: \
 source1.d \
 source2.d \

source1.d:

source2.d: