Improve Pipe task support in verifier

[porridge]

#1402 adds basic support for Pipe tasks to the package verifier - just enough for it not to break on every package which uses {{ .Pipes.... }}.

What would you like to be added:

The verifier should be able to check more things about Pipe tasks, such as:

• whether a given key is reachable from a template (I guess this depends on whether a Pipe task runs in the same plan but before a given template).
• whether a pipe task definition is complete and consistent

Why is this needed:

See #1402 (comment)