Rename ruff_python crate to ruff_python_stdlib (#3354)

In hindsight, `ruff_python` is too general. A good giveaway is that it's actually a prefix of some other crates. The intent of this crate is to reimplement pieces of the Python standard library and CPython itself, so `ruff_python_stdlib` feels appropriate.

crates/ruff_python_stdlib/src/identifiers.rs

@@ -0,0 +1,49 @@
+/// Returns `true` if a string is a valid Python identifier (e.g., variable
+/// name).
+pub fn is_identifier(s: &str) -> bool {
+    // Is the first character a letter or underscore?
+    if !s
+        .chars()
+        .next()
+        .map_or(false, |c| c.is_alphabetic() || c == '_')
+    {
+        return false;
+    }
+
+    // Are the rest of the characters letters, digits, or underscores?
+    s.chars().skip(1).all(|c| c.is_alphanumeric() || c == '_')
+}
+
+/// Returns `true` if a string is a private identifier, such that, when the
+/// identifier is defined in a class definition, it will be mangled prior to
+/// code generation.
+///
+/// See: <https://docs.python.org/3.5/reference/expressions.html?highlight=mangling#index-5>.
+pub fn is_mangled_private(id: &str) -> bool {
+    id.starts_with("__") && !id.ends_with("__")
+}
+
+/// Returns `true` if a string is a PEP 8-compliant module name (i.e., consists of lowercase
+/// letters, numbers, and underscores).
+pub fn is_module_name(s: &str) -> bool {
+    s.chars()
+        .all(|c| c.is_lowercase() || c.is_numeric() || c == '_')
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::identifiers::is_module_name;
+
+    #[test]
+    fn test_is_module_name() {
+        assert!(is_module_name("a"));
+        assert!(is_module_name("abc"));
+        assert!(is_module_name("abc0"));
+        assert!(is_module_name("abc_"));
+        assert!(is_module_name("a_b_c"));
+        assert!(is_module_name("0abc"));
+        assert!(is_module_name("_abc"));
+        assert!(!is_module_name("a-b-c"));
+        assert!(!is_module_name("a_B_c"));
+    }
+}