Auto merge of rust-lang#6756 - smoelius:clippy_utils, r=flip1995,obi-obk

Factor out `clippy_utils` crate

As discussed in rust-lang/rust-clippy#6746, this PR factors out `clippy_lints::utils` as its own crate, `clippy_utils` .

This change will allow `clippy_utils` to be used in lints outside of Clippy.

There is no plan to publish this crate on `crates.io` (see rust-lang/rust-clippy#6746 (comment)). Dependent crates should obtain it from GitHub.

changelog: Factor out `clippy_utils` so it can be used by external tools (not published)

.gitignore

@@ -18,6 +18,7 @@ out
 *Cargo.lock
 /target
 /clippy_lints/target
+/clippy_utils/target
 /clippy_workspace_tests/target
 /clippy_dev/target
 /rustc_tools_util/target

clippy_lints/Cargo.toml

@@ -18,6 +18,7 @@ edition = "2018"
 
 [dependencies]
 cargo_metadata = "0.12"
+clippy_utils = { path = "../clippy_utils" }
 if_chain = "1.0.0"
 itertools = "0.9"
 pulldown-cmark = { version = "0.8", default-features = false }
@@ -38,4 +39,4 @@ syn = { version = "1", features = ["full"] }
 [features]
 deny-warnings = []
 # build clippy with internal lints enabled, off by default
-internal-lints = []
+internal-lints = ["clippy_utils/internal-lints"]