Add 'runtime' dependencies

[Andrey Smirnov]

Proposal: add runtime flag to stage::

dependencies:
   - stage: m4
   - stage perl:
     runtime: yes

By default, all the dependencies are 'build' dependencies, i.e. they're pulled for the build. If dependency is marked as runtime, then if one stage is pulled in as a dependency, all of its runtime dependencies are pulled in as well.

Examples: automake requires perl (and m4 iirc). So if automake is pulled in as build dependency, it should always pull perl and m4, as automake doesn't run without it.