[GR-64145] Various Layer{Create,Use} option fixes.

[graalvmbot]

• Consistent checking for Layer{Create,Use} enablement
• Ensure layer-bundle file gets written into image output directory
• Enlist layer-bundle file in output artifacts list
• Uniform access to SubstrateOptions.Path value
• Derive layer filename from image name if not specified
• Update NativeImageLayers.md spec to incorporate recent changes

Now
native-image -H:+UnlockExperimentalVMOptions -H:LayerCreate=,module=java.base,package=java.net.http -o foo/bar/base-layer
results in

Build artifacts:
 /home/user/foo/bar/base-layer.nil (image_layer_bundle)
 /home/user/foo/bar/base-layer.so (image_layer)
 ...
========================================================================================================================
Finished generating 'base-layer' in 1m 3s.