Require all paths passed to `ShouldRun::paths` to exist on disk

This has two benefits:
1. There is a clearer mental model of how bootstrap works. Steps correspond to paths on disk unless it's strictly impossible for them to do so (e.g. dist components).
2. Bootstrap has better checks for internal consistency. This caught several issues:
  - `src/sanitizers` doesn't exist; I changed it to just be a `sanitizers` alias.
  - `src/tools/lld` doesn't exist; I removed it, since `lld` alone already works.
  - `src/llvm` doesn't exist; removed it since `llvm` and `src/llvm-project` both work.
  - `src/lldb_batchmode.py` doesn't exist, it was moved to `src/etc`.
  - `install` was still using `src/librustc` instead of `compiler/rustc`.
  - None of the tools in `dist` / `install` allowed using `src/tools/X` to build them. This might be intentional - I can change them to aliases if you like.

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