Hi,
from my point of view this discussion mixes two or three different questions:
* Do we want core to regularly use external libraries (which are installed via one git submodule or composer)? * Should Swiftmailer be used in core? * (How) Do we want to handle this for plain git cloning to be usable?
I think the first question is sort of answered, although maybe not everybody committed on that.
The second question is something different entirely. From my point of view it doesn't make any sense to discuss this here, because (considering how we IMHO answered the first question) we will have a required dependency eventually, even if that is not going to be Swiftmailer.
The third question is what Tim actually asked, and it's an interesting, technical question we should focus on. I like Tim's suggestion of having an optional git submodule. The installer could then show an error if vendor/ is missing proposing to either run "composer install" or "git submodule update --init". Do we also have to consider composer.lock?
Regards, Adrian