M-x package-vc-install RET https://codeberg.org/martianh/mastodon.el RET

This will fetch the package sources, but since the actual code is
located in a "lisp" subdirectory, none of the actual files will be added
to the load-path, making it appear as tough nothing were installed.

This could be fixed by adding a heuristic to package-vc--unpack-1 that
checks if there is a ./lisp/ directory (with .el files?) and that would
add that to the load-path.  Are there any other directory names or
structures that should be considered as well?  Is it common for packages
to have loadable files in both the root directory of a repository and a
sub-directory?

I could imagine that this would be enough to resolve the issue: