Please update Declt's source to github only.

The previous source(s) will disappear soon.
I believe that `source.txt` should be updated to read `git https://github.com/didierverna/declt.git`.
You can compare with https://github.com/didierverna/quicklisp-projects/tree/declt-fix. I did not create a pull request since you don't seem to want that.