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.

Empieza por el issue de GitLab enlazado y su descripción de Background y Task. Mapea los puntos de entrada existentes de los elementos de BuildStream antes de diseñar la ejecución de Bazel en sandbox, el manejo recursivo de dependencias, la generación de WORKSPACE, el montaje de una caché persistente y la generación de la toolchain para la plataforma de destino; se considera terminado cuando estos requisitos se admiten conjuntamente.

Escrito por el modelo de indexación a partir del texto del issue.