generate, validate: isolate gojson* dependencies.

[klihub]

Split out those few capability validation functions (LastCap(), CapValid()) which Generator depends on into a validate/capabilities subpackage of their own. This should prevent github.com/xeipuuv/gojson* from sneaking in to the dependencies of anyone who uses Generator for OCI Spec manipulation.

Those gojsonschema and related packages are often considered problematic because they do not seem to be actively maintained. At the time of this commit they received their last updates in 2018, 2019 and 2020. Excluding them from the dependency blast radius can lower the barrier for accepting/allowing import of generate.Generator.

[pohly]

Those gojsonschema and related packages are often considered problematic because they do not seem to be actively maintained.

Even without that concern it is better to not pull in additional dependencies that aren't needed. For example, compilation becomes faster.

[pohly]

@saschagrunert: thanks. Is there a chance to get this into a tagged release soon(ish)?

[saschagrunert]

@kolyshkin PTAL

[rhatdan]

@giuseppe @mrunalp PTAL

[elezar]

@saschagrunert @rhatdan what is still required to get these changes merged?