springboot Remove implicit layers support from the layer tools

We discussed this today and have a few concerns:

• We're duplicating a concept between the loader tools and the layer tools.
• It's surprising that you can use layered tools in an unlayered jar
• If we want to add a layer for jars in the same group, that won't be possible with the implicit layers as the info about the group ID isn't available.

We've concluded that we should remove the implicit layer support from the layer tools. Instead, when the index can't be found in the jar, we should fail.