Allow specifying element-plugin-wise default dependencies

Background

The topic was discussed on the mailing list last month - quick summary here:

It's annoying that we have to specify every dependency explicitly. Often, a certain type of build element will require an element from somewhere in the project throughout everything else, so we end up with a autotools.bst that's simply repeated everywhere.

This is a bit verbose, so we want to extend our composition format to allow specifying base dependencies for a plugin type on a project-wide level. The way we want to go about this is to add project.conf configuration to define this like so:

elements:
  autotools:
    - autotools.bst

Element dependency syntax is then adjusted to append to the default list of elements, allowing us to more concisely define "default" dependencies.

Task description

• Make element dependencies part of YAML composition
• Introduce default dependency syntax

Acceptance Criteria

It is possible to define default dependencies for plugins on a project-level.

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