Allow --pvp-bounds to use multiple stack.yaml configurations

This is split off from https://github.com/commercialhaskell/stack/issues/1568, which focused on having multiple configurations in a single stack.yaml file.

As @sjakobi suggests, it's quite reasonable to still have a --pvp-bounds flag which uses multiple configs:

How about specifying the extra stack.yamls to consider with an optional flag to --pvp-bounds, e.g. --also-considering? Example command: stack sdist --pvp-bounds both --also-considering stack-7.8.yaml stack-8.0.yaml.

Folks would probably want to automate this via a Makefile or script.