Clean up rustc_*/Cargo.toml.

- Sort dependencies and features sections.
- Add `tidy` markers to the sorted sections so they stay sorted.
- Remove empty `[lib`] sections.
- Remove "See more keys..." comments.

Excluded files:
- rustc_codegen_{cranelift,gcc}, because they're external.
- rustc_lexer, because it has external use.
- stable_mir, because it has external use.

compiler/rustc_arena/Cargo.toml

@@ -4,4 +4,6 @@ version = "0.0.0"
 edition = "2021"
 
 [dependencies]
+# tidy-alphabetical-start
 smallvec = { version = "1.8.1", features = ["union", "may_dangle"] }
+# tidy-alphabetical-end