Rollup merge of #63036 - topecongiro:add-lib-section, r=matklad
Rollup merge of #63036 - topecongiro:add-lib-section, r=matklad

Add lib section to rustc_lexer's Cargo.toml

This is required to fix the rustc-ap-syntax build error in the recent version. The error could also be fixed on the [rustc-auto-publish](https://github.com/alexcrichton/rustc-auto-publish) side by manually adding `[lib]` section if one does not exist. The latter approach, however, may have a surprising side effect, so I am opting for a simpler solution for now.

r? @alexcrichton

graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust