RFE: Collection aware and project-local ephemeral directories

[Qalthos]

Presently, the ephemeral directory lives in XDG_CACHE_HOME:

/home/bthornto/.cache/molecule
└── extensions
    ├── default
    │   └── inventory

Following #4000, we could use the collection name when constructing the ephemeral directory. Today, if using molecule in multiple collections the ephemeral directory is shared due to the introduction of the "extensions" directory.

Additionally, it could be useful for the ephemeral directory to be local to the project, say in a .cache folder in the project root. This would aid debugging and help in containerized scenarios.

[kominoshja]

Could also be solved by #4266