[BuildStream] Proposal: Decoupling source tracking from *all* commands

[tristan maat codethink co uk (Tristan Daniël Maat)]

Hi,

We recently removed the `bst build --track` options [1] because we felt
that it was not worth maintaining a fairly complex part of the codebase
for this feature.

I've started looking at cleaning up the leftovers; and actually making
it worth it by reducing the maintenance effort and complexity
surrounding our handling of tracking.

I realized that there are two more commands that have `--track` options
- `bst workspace open` and `bst source fetch`.

Given that we have removed the `--track` flag from `bst build`, I
believe that this results in an inconsistent UI, and that the `--track`
option should be removed from these commands with similar reasoning as
it was from `bst build` - it adds too much complexity for little payoff.

Currently the only way to actually make the `bst build --track` removal
useful is to run multiple pipelines in a single invocation of
BuildStream for those two commands, but this almost seems like it would
be more complex than what we had originally.

Is there any reason these options should still exist given that we have
removed `bst build --track`?

Regards,

Tristan Maat

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[1]: https://gitlab.com/BuildStream/buildstream/merge_requests/1671