As it is, adding dependencies to your project is a bit of a pain. You go to the dependencies repo, copy the Git URL, check to see what the latest version is, go back to your project, add the dependency to your manifests dependencies array, run swift package update, generate an Xcode project or try to build, realize you forgot to add the dependency to your target(s), add it to the target(s), regenerate your Xcode project (if it applies),
and now it works! To sum it up, it requires too much back-and-forth work.
I think it would make sense to have an install command that works similarly to Go:
go get github.com/user/repo
swift package install github.com/user/repo
This would add the dependency to the manifest and one or more of its products to any of the project's targets.
We could use the latest git tag, if none is provided, have a --branch <branch> flag if you want to just pick HEAD of a given branch, and have an @<tag> suffix on package names if you want a specific tag?