only show lint messages of the current linting file

I'm working on Rust Linter, which would like to support Rust's package manager, Cargo. Cargo was designed to build a package (or project), which is a group of many source files. While linting a file inside a package, we build the full package (use Cargo). (You can't just compile only one of these files individually.) There are maybe some errors, cause by files other than the current linting one, which need to be ignored. This PR changes the behavior to only show lint messages of the current linting file.

This is a backward compatible improvement.