dune utop fails

[cemerick]

I'm using ppx_rapper in a project that builds and tests successfully, but I cannot run utop in it. dune utop test (for example) results in:

File "_none_", line 1:
Error: Module `Pg_query' is unavailable (required by `Ppx_rapper')

Pg_query is provided by the pg_query package, which is available: ppx_wrapper depends on it directly, and regular builds would fail otherwise.

I've created a repo that minimally reproduces the failure; see the README for an invocation to create a fresh switch, build, and try to run a toplevel:

https://github.com/cemerick/rapper_utop

Maybe I'm doing something wrong, but I've been stumped so far. 🤔