Remove some usages of Ruff internals in ruff_dev (#610)

ruff_dev/src/generate_source_code.rs

@@ -1,11 +1,11 @@
 //! Run round-trip source code generation on a given Python file.
 
+use std::fs;
 use std::path::PathBuf;
 
 use anyhow::Result;
 use clap::Args;
 use ruff::code_gen::SourceGenerator;
-use ruff::fs;
 use rustpython_parser::parser;
 
 #[derive(Args)]
@@ -16,7 +16,7 @@ pub struct Cli {
 }
 
 pub fn main(cli: &Cli) -> Result<()> {
-    let contents = fs::read_file(&cli.file)?;
+    let contents = fs::read_to_string(&cli.file)?;
     let python_ast = parser::parse_program(&contents, &cli.file.to_string_lossy())?;
     let mut generator = SourceGenerator::new();
     generator.unparse_suite(&python_ast)?;