Support recipes without a version and without a conandata.yml

[foundry-markf]

For example, this could be a recipe for a package whose version is procedural from a source control tag.

Suggestion:

• if a recipe has a version attribute, use that, else
• if a conandata.yml, offer a choice of those present (assuming it's parsable), else
• provide ability to enter a version number

[markfinal]

This is what you currently see when there is no version attribute and no conandata.yml. You are unable to proceed to load the recipe in the UI.