consider commandlet dependencies

[hohwille]

Currently when a commandlet is invoked manually to setup (e.g. not included in DEVON_TOOLS) and it may have dependencies then the installation fails.
Examples for dependencies:

• mvn -> java
• ng -> npm -> node

We should therefore carefully consider if a commandlet shall be aware of its dependency and invoke setup on the dependency before.
Then devon mvn setup would automatically also invoke devon java setup.

Are there drawbacks of this approach that I am issing?

[hohwille]

Done with PR #168