allow installation of packages from repository server

[line-o]

Enhancement

At the moment xst only allows installation of packages that are locally available. While this is useful for local build artefacts there are plenty of pre-built packages available at the public package repository.

It would be handy if xst would allow to install any of them without having to download them first.

xst package install --from-repo [--repo-url] <packages..>

The XQuery implementation might end up just being a call to repo:install-and-deploy#2 and some error handling wrapped around it.

related to #13 (to query 3rd party repositories)

Alternatives

No response

[line-o]

This is a very different approach than #80 because the exist-instance must be able to access the repository.

[line-o]

@joewiz I hope this makes it a little clearer what the goal of #80 is in contrast.