Remove git2 dependency

[federico-terzi]

Currently espanso uses the git2 library to download packages, but this comes with a few downsides.
In particular, on macOS the user is required to install openssl, which is inconvenient in some circumstances.

By removing this dependency, espanso could be easier to install in many circumstances, especially with the upcoming features.

The packages will be then installed via HTTPS using the (already available) alternative method. If for any reason, you would still prefer to install packages through git, let me know (in that case I would probably leverage on the command line git command rather than the library though).