Implicit layer versioning may not be consistent across platforms

[ncoghlan]

All layer versioning metadata is currently tracked in platform specific files.

Since that tracking includes the nominal version of implicitly versioned layers, those version numbers may end up differing across platforms if:

• a new platform is added to the stack build (as its version will automatically start at one with no current ability to override it)
• one platform is rebuilt multiple times for incremental changes, and then the remaining platforms are only rebuilt once for the full set of changes
• a transitive dependency change only updates the layer lock on a subset of platforms

It's not clear that there is a good alternative to this, though, so it may be that the answer is that if this isn't OK for a given use case, that use case needs to use explicit layer versioning instead.

That conclusion would modify the design question to be whether it would be worth making explicit layer versioning slightly easier to use by adding a version=NNN integer field to the layer specification.