Create a bazel build element

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/529)
In GitLab by [[Gitlab user @edbaunton]](https://gitlab.com/edbaunton) on Jul 30, 2018, 16:20

## Background

Create an element that provides support for building with `bazel`.

Specifically, a bst would declare itself of kind `bazel` and BuildStream would take care of generating the `WORKSPACE` file by recursively scanning the dependencies of the element being built.

## Task description

* `bazel` would be executed inside the sandbox (not using a host tool)
* The workspace file would identify the built sources to consume with `new_local_repository` rules; if it encounters a dependency of type `bazel` it would stage the sources rather than the binaries and consume the `BUILD` file to leverage `bazel`.
* `bazel` would need access to a persistent disk cache (use `_set_mount_source`?) between BuildStream runs so as to take advantage of its built in caching. The `bazel` server will actually be torn down between runs but it is said to serialize its state between runs so the startup time will no be prohibitive.
* Toolchain generation: we need to generate correct toolchain for the target platform and have that be considered as part of the key for the element.

Commencez par l’issue GitLab liée et sa description de Background et Task. Cartographiez les points d’entrée existants des éléments BuildStream avant de concevoir l’exécution de Bazel en sandbox, la gestion récursive des dépendances, la génération de WORKSPACE, le montage d’un cache persistant et la génération de la toolchain pour la plateforme cible ; le travail est considéré comme terminé lorsque ces exigences sont prises en charge ensemble.

Rédigé par le modèle d'indexation à partir du texte de l'issue.