Allow users to `add` a "legacy" package without `paket.toml`

Background: I would like to use https://github.com/catppuccin/fish, which is a simple "theme-only" package, but currently it just says:

Error: paket.toml file was not found on package fish or inaccessible.

Since paket is relatively a new thing, would be great not requiring upstream changes for "simple" packages like this one (a "info" log suggesting such change can be added though)