Make main_output public API

[keith]

This API allows creating shared libraries with custom filenames, not
just the default pattern. This is required to write a rule that produces
python native extensions which have entirely different output naming
formats than standard shared libraries.

This API has been around for years now so I assume it's well understood
enough and there isn't a clear alternative.

[keith]

@trybka can you take a look at this one? This is what I was mentioning recently

[trybka]

I'm inclined to approve this, but I am just soliciting one round of internal discussion to see if I get significant push back.

From my POV, callers use this all the time, they just use weird hacks to get around the allowlist. We might as well accept Hyrum's law and this is just part of our API.

From a pure API standpoint it makes sense to me to say, "you can set the output name" (just as you would with clang -o foo.exe and I don't think it is reasonable to force it to be derived from the target name and executable type.