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/lib.rs

@@ -0,0 +1,16 @@
+/*!
+An implementation of `grep-matcher`'s `Matcher` trait for
+[PCRE2](https://www.pcre.org/).
+*/
+
+#![deny(missing_docs)]
+
+extern crate grep_matcher;
+extern crate pcre2;
+
+pub use error::{Error, ErrorKind};
+pub use matcher::{RegexCaptures, RegexMatcher, RegexMatcherBuilder};
+pub use pcre2::{is_jit_available, version};
+
+mod error;
+mod matcher;