Package installations from several sources

[line-o]

New

• xst package install github-release <abbrev> [<version>] install a package from a GitHub release
• xst package install registry <package> install a package from a package registry (also known as public-repo)
• xst package install local <packages..> install packages from a local files

Breaking change

Calls to xst package install <packages..> need to be xst package install local <packages..>