Improve Spack lecture

The following points need to be worked upon in the Spack lecture:

• Explicitly state that Spack actually builds packages from source, and also its dependencies from source. This is the principle difference to what apt or pip does.
• Make it clear that the checksum in the versions in package.py is used to verify if the sources are actually the ones which were originally released. Avoid calling the checksum an identifier.
• The difference between using Spack as a user and as a developer of a package needs to be more explicitly stated by adding a user task in the exercise.