Fix permission issues during layer handling

[Malax]

When a layer directory contains files/directories that cannot be deleted based on their permissions, layer handling might fail with hard to debug errors that lack the necessary context.

This PR fixes this in two ways. First, a dedicated DeleteLayerError has been introduced to add more context to potential IO errors. WriteLayerError and ReadLayerError existed already. Secondly, since the buildpack process usually has the necessary permissions to change the permissions of the layer contents, libcnb now attempts to fix the permissions of the layer contents before attempting to delete the layer.

I also added a new test-buildpacks directory that contains buildpacks like the ones in the examples directory. The buildpacks within are meant as integration tests for libcnb itself. The directory contains a README explaining the intent of that directory.

Changelog

• Fix permission issues during layer handling when the layer contains read-only directories.
• Add explicit DeleteLayerError to provide more context when debugging layer handling problems.

Fixes #475, GUS-W-11467784

[colincasey]

👍 i like the direction of the test-buildpacks here