coq: update libdir paths a la ocaml-findlib

See coq/coq#15663 (comment) and
coq/coq#15635 (comment) and
coq/coq#18439.  Coq lib files must be
installed to the same location as ocaml-findlib lib files in order for
plugins to be buildable.

An alternative as per
coq/coq#15452 (comment) might be
```diff
diff --git a/Formula/c/coq.rb b/Formula/c/coq.rb
index 580810fc3ed..ed8f82f230b 100644
--- a/Formula/c/coq.rb
+++ b/Formula/c/coq.rb
@@ -36,7 +36,7 @@ class Coq < Formula
     ENV.prepend_path "OCAMLPATH", Formula["ocaml-findlib"].opt_lib/"ocaml"
     system "./configure", "-prefix", prefix,
                           "-mandir", man,
-                          "-libdir", HOMEBREW_PREFIX/"lib/ocaml/coq",
+                          "-libdir", HOMEBREW_PREFIX/"lib/coq",
                           "-docdir", pkgshare/"latex"
     system "make", "dunestrap"
     system "dune", "build", "-p", "coq-core,coq-stdlib,coqide-server,coq"
@@ -44,9 +44,12 @@ class Coq < Formula
                               "--mandir=#{man}",
                               "--libdir=#{lib}/ocaml",
                               "coq-core",
-                              "coq-stdlib",
                               "coqide-server",
                               "coq"
+    system "dune", "install", "--prefix=#{prefix}",
+                              "--mandir=#{man}",
+                              "--libdir=#{lib}/coq",
+                              "coq-stdlib"
   end

   test do
```

Bump revision on coq to force a rebuild.

Formula/c/coq.rb

@@ -5,6 +5,7 @@ class Coq < Formula
   sha256 "17e5c10fadcd3cda7509d822099a892fcd003485272b56a45abd30390f6a426f"
   license "LGPL-2.1-only"
   head "https://github.com/coq/coq.git", branch: "master"
+  revision 1
 
   livecheck do
     url :stable
@@ -35,11 +36,13 @@ def install
     ENV.prepend_path "OCAMLPATH", Formula["ocaml-findlib"].opt_lib/"ocaml"
     system "./configure", "-prefix", prefix,
                           "-mandir", man,
+                          "-libdir", HOMEBREW_PREFIX/"lib/ocaml/coq",
                           "-docdir", pkgshare/"latex"
     system "make", "dunestrap"
     system "dune", "build", "-p", "coq-core,coq-stdlib,coqide-server,coq"
     system "dune", "install", "--prefix=#{prefix}",
                               "--mandir=#{man}",
+                              "--libdir=#{lib}/ocaml",
                               "coq-core",
                               "coq-stdlib",
                               "coqide-server",