improve dependency terminology

[grahamdyson]

Eunice currently uses the term "depends upon". The intention of using this terminology was to avoid ambiguity in what direction the dependency being defined has. This terminology is used in YAML.

- id: item 1
  dependsUpon: item 2
- item 2

I think this can be simplified and made easier to remember.

• The word "upon" is unnecessarily formal and doesn't add more meaning than just "on".
• "depends upon" or even "depends on" is difficult to differentiate between singular and multiple.
• It is not possible in Eunice YAML to represent dependencies in the other direction.
• There is a convention in software to describe dependencies in this direction just as "dependencies". In other projects (e.g. NPM) dependencies in the other direction are defined as being "dependents".

In YAML "dependsUpon" could be replaced with just "dependencies".