Rework Symbol Support

[Gankra]

I've disabled all symbols pending this redesign of the feature, due to several issues:

• People's builds were failing to find/build symbols and dying due to us messing up or them setting flags like strip=true
• The resulting output had dubious usefulness due to properly handle platform-specific hyphen-remapping #116

I propose two fixes:

Fix 1: We shouldn't consider it a hard error to fail to generate a pdb

This creates an awkward situation with manifest --artifacts=all, due to a lack of #45. I think rather than "solving" #45 we should just add an optional: true field to dist-manifest.json indicating the symbols artifacts are ok to fail. Then we should also change cargo dist build to silently remove optional: true entries from its output if they indeed failed (preventing CI from trying to upload nothing). Someone reading dist-manifest.json then just needs to accept that they might fail to fetch those (seems fine).

Fix 2: We should unconditionally wrap every kind of symbol file in a zip

That will allow us to preserve the name rustc/cargo told us was the Right one. This will handle the most important aspects of #116, which is getting the symbols to a server/user with the right name. The consumer of such a zip just needs to understand they have to properly spider the folder. Ideally each folder will contain one entry.

I'm not 100% sure what to name the zips and the folders they contain. This is most awkward for blah.dSYM because we want blah-v1.0.0-x86_64-apple-darwin.dSYM.tar.xz to contain a dir containing the .dSYM but if we just name the dir blah-v1.0.0-x86_64-apple-darwin.dSYM things might freak out and think it's an actual dSYM (because those are dirs!). Maybe just call the inner dir blah-v1.0.0-x86_64-apple-darwin-dSYM? Are tar and zip happy with that kind of rename? I think so?