Replace custom 'activate' method and symlink injection with a `MELTANO_PATH` env var - or a modified `PATH` - for child processes

In #6468, we are improving our interop for how Meltano is able to create symlinks which ensure it will be made available to child processes and processes like Airflow which will want to callback or invoke Meltano directly.

Now that we have the EDK and a growing set of best practice paradigms, this seems like a problem we could solve in a better way than the symlink approach.

Two possible options for working around the symlink approach:

1. Pass to child processes a MELTANO_PATH env var which points to the Meltano executable.
2. Before invoking a child process, prefix the child process’s PATH env var with the path to the Meltano executable.

These options also have the nice benefits of:

1. Working well within containers, where Meltano is expected to be installed but may not be in an obvious location.
2. Working well in contexts where multiple versions of Meltano might be installed.

[DouweM]

@aaronsteers The symlink existed only for the benefit of https://gitlab.com/meltano/files-airflow/-/blob/master/bundle/orchestrate/dags/meltano.py#L38, so indeed adding it to the PATH or exposing it through MELTANO_PATH/EXECUTABLE should be sufficient.

[aaronsteers]

(Since I/we don’t have a lot of background info here, perhaps good to phone a friend with more historical context 📞 😄.)

Seems pretty niche upon reflection, so I suppose there isn’t much reason for the symlink save for its legacy usage.

@DouweM - We postulate above that the symlink’s primary benefit is for subprocess/sideprocesses which need to find the meltano executable - and that similar functionality could be replaced by simply ensuring meltano is on the PATH or else explicitly pointed to in a future MELTANO_PATH env var which would be leveraged by meltano-integrated tools when it is found in context.

By chance do you have any additional historic or technical context for the need for the symlink which we haven’t covered here?