Ability to manually optimize build graph

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/942)
In GitLab by [[Gitlab user @nanonyme]](https://gitlab.com/nanonyme) on Mar 3, 2019, 14:21

This is mostly a request for comments at this point. There's some exceptional build items (read: take very long to compile) in freedesktop-sdk like LLVM that are the reverse dependencies of many things and as a consequence you'd want to to start compiling them as early as possible. In other words, not all elements are really equal.

Should BuildStream expose some tools (eg element priority) to projects to be able to have more control over element build order? Is it already in current element model possible to affect that without adding artificial dependencies?

Start by reviewing BuildStream's current element model and build-graph scheduling behavior, as well as the original GitLab issue. Determine whether element priority or another supported mechanism can influence build order without artificial dependencies. Done should mean the project has a defined, implementable way to prioritize exceptional long-running elements.

Written by the indexing model from the issue text.