sbt's shell provided completion only for keys that were relative to a
defined project, but didn't provide completion for keys that belong to
the build definition only.
This commit fixes this issue by defining a new kind of `Parser` (from
which completions are generated) which runs its input simultaneously on
distinct parsers. We now define a parser for project-level keys and
another parser for build-level keys. These two parsers are eventually
combined, and we get the completions of both parsers.
Fixessbt/sbt#2460