[BuildStream] Feature proposal: Add --build option to bst checkout

[Chandan Singh <chandan chandansingh net>]

Hi,

I was considering adding a `--build` option to `bst checkout` that would alter
the behavior of this command similar to how `--build` works in `bst shell`.
So, instead of checking out an element (and its runtime dependencies),
BuildStream would checkout its build dependencies and sources instead. This
checkout will look the same as the file system tree of the build sandbox.

`bst shell --build` is very useful in debugging a build but there are some
scenarios where it is not quite suitable. Here are some examples where
`checkout --build` might be more useful:

- if BuildStream is running as a remote execution client, `shell --build` will
  be unusable

- such a checkout can easily be imported into other systems, like Docker, which
  can be useful for debugging purposes

- in certain cases, it will allow users to involve host tools for debugging

Proposed changes
================

- Add `--build` option to `bst checkout`

I think this option should also be mutually exclusive with `--deps [none,run]`
as that makes sense in the context of checking out built artifacts, whereas
`--build` makes sense in the context of checking out the sources. We can also
consider adding `--deps build` option instead but in my opinion that may be
more confusing for the user.

That's it. Thoughts?

Cheers,
Chandan