support blind-translated YAML mapping for galaxy deps

[nitzmahone]

• simplify docs accordingly

[nitzmahone]

If the "blind-translated" in the title wasn't enough, we should probably be explicit somewhere in the docs about the fact that by design, we do zero validation of the structure of this YAML beyond requiring that it's a dictionary- we just splat it into a requirements.yml in the root of the build context.

We don't builder to be tightly-coupled to Galaxy CLI's schema, so the fact that it's "YAML-in-YAML" is merely a convenience- we shouldn't rely on this going forward (eg, even though inlining things like ignores here might be really easy and tempting, we should probably resist the urge since it would require us to more fully understand and rewrite the contents of the galaxy dict)...

[Shrews]

If this is going to be the preferred format, we should change demo/execution-environment.yml too.

[cidrblock]

TY. Even without validation against the galaxy expected syntax, the simple fact this is now a structured list will help people spot issues.