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_macros/Cargo.toml

@@ -7,7 +7,9 @@ edition = "2021"
 proc-macro = true
 
 [dependencies]
-synstructure = "0.13.0"
-syn = { version = "2.0.9", features = ["full"] }
+# tidy-alphabetical-start
 proc-macro2 = "1"
 quote = "1"
+syn = { version = "2.0.9", features = ["full"] }
+synstructure = "0.13.0"
+# tidy-alphabetical-end