Log when cache cleared and why

[schneems]

In #47 I noticed that we're not logging when the cache is cleared or why. This is important for buildpack authors so we can visibly see unexpected behavior (in development and debugging for support tickets) as well as for customers so they can better understand what's going on.

Cache invalidation is a famously hard thing, we should set up our users and future buildpack maintainer for success.

I would like:

• Logging when metadata is invalid that states why it's invalid: In the event that a buildpack author made a change that accidentally cleared the cache or if we're somehow writing data that cannot be read by accident then we would want to know and fix it.
• When metadata changes, state which metadata changed and the values.

One option forward is for me to get approval on https://github.com/heroku/buildpacks-ruby/blob/3f3e21ae670d75b8cc0dd11fc58c95769828ed1c/buildpacks/ruby/src/layers/shared.rs and if you like that API, I could move it to the Ruby's "commons" where it can be re-used.