repo: move all source code in crates directory

The top-level listing was just getting a bit too long for my taste. So
put all of the code in one directory and shrink the large top-level mess
to a small top-level mess.

NOTE: This commit only contains renames. The subsequent commit will
actually make ripgrep build again. We do it this way with the naive hope
that this will make it easier for git history to track the renames.
Sigh.

crates/pcre2/src/error.rs

@@ -0,0 +1,59 @@
+use std::error;
+use std::fmt;
+
+/// An error that can occur in this crate.
+///
+/// Generally, this error corresponds to problems building a regular
+/// expression, whether it's in parsing, compilation or a problem with
+/// guaranteeing a configured optimization.
+#[derive(Clone, Debug)]
+pub struct Error {
+    kind: ErrorKind,
+}
+
+impl Error {
+    pub(crate) fn regex<E: error::Error>(err: E) -> Error {
+        Error { kind: ErrorKind::Regex(err.to_string()) }
+    }
+
+    /// Return the kind of this error.
+    pub fn kind(&self) -> &ErrorKind {
+        &self.kind
+    }
+}
+
+/// The kind of an error that can occur.
+#[derive(Clone, Debug)]
+pub enum ErrorKind {
+    /// An error that occurred as a result of parsing a regular expression.
+    /// This can be a syntax error or an error that results from attempting to
+    /// compile a regular expression that is too big.
+    ///
+    /// The string here is the underlying error converted to a string.
+    Regex(String),
+    /// Hints that destructuring should not be exhaustive.
+    ///
+    /// This enum may grow additional variants, so this makes sure clients
+    /// don't count on exhaustive matching. (Otherwise, adding a new variant
+    /// could break existing code.)
+    #[doc(hidden)]
+    __Nonexhaustive,
+}
+
+impl error::Error for Error {
+    fn description(&self) -> &str {
+        match self.kind {
+            ErrorKind::Regex(_) => "regex error",
+            ErrorKind::__Nonexhaustive => unreachable!(),
+        }
+    }
+}
+
+impl fmt::Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match self.kind {
+            ErrorKind::Regex(ref s) => write!(f, "{}", s),
+            ErrorKind::__Nonexhaustive => unreachable!(),
+        }
+    }
+}