Extract init_env_logger to crate
I've been doing some work on rustc_ast_pretty using an out-of-tree main.rs and Cargo.toml with the following:
```toml
[dependencies]
rustc_ast = { path = "../rust/compiler/rustc_ast" }
rustc_ast_pretty = { path = "../rust/compiler/rustc_ast_pretty" }
rustc_span = { path = "../rust/compiler/rustc_span" }
```
Rustc_ast_pretty helpfully uses `tracing::debug!` but I found that in order to enable the debug output, my test crate must depend on rustc_driver which is an enormously bigger dependency than what I have been using so far, and slows down iteration time because an enormous dependency tree between rustc_ast and rustc_driver must now be rebuilt after every ast change.
I pulled out the tracing initialization to a new minimal rustc_log crate so that projects depending on the other rustc crates, like rustc_ast_pretty, can access the `debug!` messages in them without building all the rest of rustc.
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