Is there such thing as "active dry sourdough-variety yeast"? Apologies in advance if this is a stupid question, or if it doesn't sound coherent, but: Is it possible to buy sourdough yeast, ready to use? 

You can easily find "active dry yeast" for sale in supermarkets. I realize this regular bread yeast is a different variety from your typical sourdough yeast. 

But can the sourdough type of yeast (I guess it is referred to as "wild yeast"?) be packaged in a way that you can just add it to your dough whenever you feel like baking, and not have to maintain a starter in a special container, with all the fussy feeding rituals, and worries about its health and environmental conditions, etc.? ... I imagine taking care of the starter must be half the fun, but people are always trying to make a buck by selling you convenience, so I wonder if you can buy little packets of "active dry sourdough yeast". 

Or does wild yeast have some characteristic that makes it impossible to package it up neatly for sale? Or is there some other reason I haven't seen this product, even on the internet? ... Or do I just need to open my eyes, and I will see it in every market?
No, I really don't think you can have "sourdough yeast".

If you want sourdough without the "fuss", you can look up sourdough flavor additive and see what works for you - one here, and here, just for examples.

The thing is, sourdough starter isn't just yeast, it is a complex community - including several strains of wild yeast and bacteria, and a complexity that comes from aging. Yeasts can be packaged up - though balancing several strains is tricky, especially if you want consistency - but the bacteria is a lot trickier, especially since conditions which support bacteria are easy to contaminate with the wrong strains, and you would want the product quick - so not taking the time for the bacteria to establish a strong culture, which leaves it more vulnerable to said contamination.

Bread yeast is, I think, basically what happened when sourdough yeast was cleaned up and regulated and made safe for storage.  I have heard of starters that began with commercial yeast - it apparently takes a while for the characteristic flavors to develop, but apparently they do. Also, I've heard extended proofing (cold proofing) gives more complex flavors somewhat similar to sourdough, so simple time is, in fact, a factor in how the taste forms - it gives time for the fermentation to work.

The fact that adding more time to yeast or dough makes a bread more complex and sour-dough like, makes me think that there's no way to build that flavor naturally and quickly - after all, there's no instant time on our shelves.