New spaces feature, and reorganized options

[cunei]

This pull request consists of two parts.

• First, it reorganizes the "build.options" section, which was confusingly similarly named to the root "options". Those options can now be directly specified inside "build". Also, the build section can be an array of sections, each with different defaults, which apply to the projects contained therein. The code has been restructured, and the handling of defaults now takes place only in clearly defined places in the code, and it is all done before any further processing is made. That also makes it much simpler to add further general defaults in the future. Also, cross-versioning selection is now per-project.
• Second it add a new "spaces" feature, by which the generated artifacts visibility can be controlled by placing them in one or more "spaces". Each project can select the space that it would like its dependencies to be rewired from, and spaces are arranged hierarchically for easier nesting. This feature makes it possible to: consolidated separate dbuild configuration files into a single file; compile projects in subsequent stages, even if they generate the same artifacts; use the same artifacts generated by one project in multiple versions of further projects. A different scala compiler can be generated and used in each space.

The two sections correspond more or less to the following two groups of commits:

• Reorganization of options and defaults: https://github.com/cunei/distributed-build/compare/6aff75c4dd8c3967d2833809cf6fa59f723173bf...dbf6ee9ccdfea99fcfbac2543523c0d528255b50?expand=1
• Implementation of spaces: https://github.com/cunei/distributed-build/compare/67104252e04d3b2b193423c040abf80bbdb047ed...128e2eb9682832a905efaff53a6261a8ce17e6a5?expand=1

The full documentation of the spaces feature is at: https://github.com/cunei/distributed-build/blob/wip-feature-spaces-and-reorganized-options/docs/src/sphinx/spaces.rst (this is a GitHub rendering of the page that will become part of the documentation website).

[cunei]

There is also a further new feature in the queue, depending on this pull request: automatic stability comparison of the results of different stages (https://github.com/cunei/distributed-build/pull/2). /cc @adriaanm

[jsuereth]

This looks good to me, just one question about a particular commit. Sorry for the long delay on review.