whatprovides - allow multiple package selection

[jmoalves]

Allow a recipe to require a capability, something that multiple packages would be able to provide.

For instance, we could have the following packages:

• node-10
• node-12
• node-18

providing the capability node.

We could have another recipe with the following dependencies:

• node

Any package listed above will be able to fulfill this.

Questions:

• How to choose one of them if none is mentioned in the dependency list?
• Should we provide a "default" declaration?
• What if we have multiple "defaults"?

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This issue is something like the whatprovides capabilty in rpm.
http://ftp.rpm.org/max-rpm/s1-rpm-query-parts.html#S3-RPM-QUERY-WHATPROVIDES-OPTION