Implement strict priority for channels

[wolfv]

We don't prioritize channels yet. We should have a flag in the solver that toggles strict priority.

Strict priority means that if a package name is available in a higher priority repository it is ignored in lower-priority repositories.

If there are two repositories, A & B and A is added with higher priority, only numpy from package A is going to be considered even if a higher version exists in B.

[baszalmstra]

This has been implemented in #385