move `model.bom.Bom.validate()` to `validation.models`

[jkowalleck]

now that a package validation was established via #432,
t should be possible to move the data model validation there, too.\

data model validation is getting more and more complex #453
and should be encapsulated and could be devided.

[saquibsaifee]

This is just moving Bom.validate() to validation.model as-is and update the references, or do you want it refactored into smaller functions?

[jkowalleck]

This is just moving Bom.validate() to validation.model as-is and update the references, or do you want it refactored into smaller functions?

existing public API shall be deprecated and something more advanced shall be provided.
details may be discussed