Bazel: Add aquery configuration() function

[kastiglione]

Description of the problem / feature request:

When using aquery, it would be convenient to limit the output based on the action's configuration.

Feature requests: what underlying problem are you trying to solve with this feature?

For an iOS target, running bazel aquery 'deps(some/target)' will result in many iOS targets, but also some targets that are not iOS. One way to distinguish iOS targets from others is by configuration. Locally, I also see actions for apl-darwin_x86_64-fastbuild and darwin-fastbuild.

It would be convenient if I could write the query as:

bazel query 'configuration(ios-x86_64, deps(some/target))'

[gregestren]

I like this idea. I also think it should integrate with cquery's concept of configuration: https://docs.bazel.build/versions/master/cquery.html#config

In other words, cquery and aquery should share the same language for configuration as much as possible.

[gregestren]

Also note that config right now only takes host (for the host configuration), target (for whatever's set by the command-line flags), and a hash of a complete configuration (retrievable via bazel config or bazel cquery). It's not currently savvy enough to take a single flag setting.

[meisterT]

cc @joeleba