Deny unreachable-pub

It's very useful when `pub` is equivalent to "this is crate's public
API", let's enforce this!

Ideally, we should enforce it for local `cargo test`, and only during
CI, but that needs https://github.com/rust-lang/cargo/issues/5034.

crates/base_db/src/input.rs

@@ -12,9 +12,7 @@ use cfg::CfgOptions;
 use rustc_hash::{FxHashMap, FxHashSet};
 use syntax::SmolStr;
 use tt::TokenExpander;
-use vfs::{file_set::FileSet, VfsPath};
-
-pub use vfs::FileId;
+use vfs::{file_set::FileSet, FileId, VfsPath};
 
 /// Files are grouped into source roots. A source root is a directory on the
 /// file systems which is watched for changes. Typically it corresponds to a