remove NNPACK and move ForwardDiff to an extension

[avik-pal]

• NNPACK currently cannot be used (has been so since the last minor release). See NNPACK convolution issue #203 (comment). I don't see why they need to be left here
• Moves ForwardDiff to an extension (any reason why it was left to be loaded via Requires?)
• As a consequence of the above 2, we can remove the dep on Requires
• Also Pkg.jl was imported for some reason 😰

[ToucheSir]

Thanks. I think what happened is that the 1.9 + extensions + Enzyme upgrades took up a lot of energy, leaving none left for cleanup. NNPACK was also a problem because the test coverage was never fixed after it broke a while ago. Either way, we can probably get away with this in a minor release.