`lx add` and `lx remove` for git dependencies

[mrcjkb]

#644 introduces basic support for git dependencies.
It would be nice to be able to lx add and lx remove them.

If specying a dependency without a version requirement, e.g. lx add "github:owner/repo", then lux will need to checkout the repository and get the current revision or latest semver tag.

[vhyrro]

sounds good, we might want to gate it behind --git like we do --test, --build etc.

[mrcjkb]

sounds good, we might want to gate it behind --git like we do --test, --build etc.

Either that, or we create Alternative style clap arg parsers, like we do with lx pack:

#[derive(Debug, Clone)]
pub enum PackageOrRockspec {
    Package(PackageReq),
    RockSpec(PathBuf),
}

impl FromStr for PackageOrRockspec {
    type Err = eyre::Error;

    fn from_str(s: &str) -> Result<Self, Self::Err> {
        let path = PathBuf::from(s);
        if path.is_file() {
            Ok(Self::RockSpec(path))
        } else {
            let pkg = PackageReq::from_str(s).map_err(|err| {
                eyre!(
                    "No file {0} found and cannot parse package query: {1}",
                    s,
                    err
                )
            })?;
            Ok(Self::Package(pkg))
        }
    }
}