Renaming architecture to layer

[perryqh]

Background

It was decided that architecture isn't a great name for a violation because most of the violation types can be seen as architecture violations.

Changes

A new violation type, layer, is added. architecture should be considered deprecated.

	Layer Violation	Architecture Violation
Status	New	Deprecated
packwerk.yml key	layers	architecture_layers
package.yml enforcement	enforce_layers	enforce_architecture
package_todo violation label	layer	architecture

Release plan

• publish this PR as v0.2.0. It offers backward compatibility with the now deprecated architecture violation
• a subsequent PR will completely remove architecture violation support
• gems that rely on the architecture naming will get updated
  • pack_stats
  • packs
  • parse_packwerk
  • visualize_packs
• https://github.com/alexevanczuk/packs will also need a PR that that mimics this PR's functionality.