Support optional dependencies

[dlang]

Support optional dependencies

[simendsjo]

Somewhat related: A package could also have a post-install message to note the user of important things (like Archs PKGBUILD for instance: https://wiki.archlinux.org/index.php/PKGBUILD#install)

[mdondorff]

Optional dependencies can now be specified using the fomat
"packageId": {"version": "4.7.1", "optional": true}

If an optional package has been found already installed on the system, dub will update it. The VisualD project generator will use and include these optional dependencies.

Open tasks:

• Add Version identifiers as specified by @s-ludwig
• Adopt other generators to understand optional dependencies (rdmd, build, monod)
• Add documentation to the dub registry package-format page

[mdondorff]

whoops

[s-ludwig]

• Add Version identifiers as specified by @s-ludwig
• Adopt other generators to understand optional dependencies (rdmd, build, monod)
• Add documentation to the dub registry package-format page

I'd say that by now all those points are implemented.