[build2] Packaging an External Library without any Versioning Scheme

[markus.pawellek at mailbox.org]

Hello,

I created a [build2 
package](https://github.com/lyrahgames/build2-happly)
that packages the single header-only library 
[hapPLY](https://github.com/nmwsharp/happly).
Unfortunately, hapPLY does not provide any versioning scheme but might 
so in the future.
Still, I would love to publish package versions to cppget.org.

I know about the 'upstream-version' feature in the 'manifest'
but do not really want to come up with an upstream-to-downstream 
version mapping
as it could break things in the future.

Instead, would it be ok to use snapshot versions for this case?
The current tagged version of the build2 package is given by

v0.1.0-a.0.20230702193747.932bd3bb80b1

In that case, `manifest` could use a dependency declaration
with a pre-release version constraint or without any version constraint.

depends: happly
depends: happly ^ 0.1.0-

What would you suggest?

Even if it is not possible to publish the package to cppget.org,
I would still like to transfer the ownership to build2-packaging
such that the 'repositories.manifest' only would need to add the git 
repository directly.

Greetings
Markus