opam-grep is an opam plugin that greps through the sources of all opam packages.
To install it, simply call:
$ opam install opam-grep
Then to use it, simply call:
$ opam grep "your regexp"
Side note: currently opam-grep will cache the sources in your cache directory ($XDG_CACHE_HOME/opam-grep), so a few GB of available disk space is most likely required.
I hope this can help core compiler and community library devs alike, to know which part of their software is used in the wild.