Add plugins for handling meson subprojects

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/1026)
In GitLab by [[Gitlab user @coldtom]](https://gitlab.com/coldtom) on May 16, 2019, 14:47

## Background

Meson has an idea of [subprojects](https://mesonbuild.com/Subprojects.html), which seem to be like git submodules but in meson. As a result meson elements can try to download these subprojects at build time and cause errors. It would be nice for BuildStream to integrate nicely with this feature. Thankfully it is possible to just depend on the subprojects as additional sources if necessary and stage them in the correct place.

## Task description

Actually adding this support could be complicated by the fact that meson is an `Element`, and not a `Source`, so doesn't touch fetch or track. I don't know how it could be achieved in a nice way for users.

## Acceptance Criteria

Meson subprojects work nicely with BuildStream.

Beginne mit dem verknüpften GitLab-Issue und der Dokumentation zu Meson subprojects, um das erwartete Verhalten zu verstehen. Untersuche, wie BuildStream Elements von Sources trennt und wie zusätzliche Sources bereitgestellt werden; fertig ist die Arbeit, wenn Meson subprojects zuverlässig funktionieren, ohne sie zur Build-Zeit herunterzuladen.

Vom Indexierungsmodell aus dem Issue-Text verfasst.