Add an alias property for packages

[DropSnorz]

Package are identified by a human-readable slug, defined as <groupId>/<packageId>

We can defined a unique alias for a package, for example wobbleizer alias will target dropsnorz/wobbleizer slug. Aliases can be used as shortcuts for command line tools, for example owlplug install wobbleizer.

• This property must be the same for all package versions
• This property should be unique for a package: alias (0,1) <=> (1) package

An alias can be defined in a meta.yaml or metadata.yaml from the package directory : ./registry/<groupId>/<packageId>