Re: meson's in-tree libpq header search order vs -Dextra_include_dirs

I took another look and got it working, after something else I'm using
insisted on installing libpq.  It's mostly no-brainer hunks like this:

-  dependencies: [frontend_code, libpq],
+  dependencies: [libpq, frontend_code],

For the new src/test/modules/test_oat_hooks/meson.build, I copied what
I'd seen in contrib modules that use libpq:

-  kwargs: pg_test_mod_args,
+  kwargs: pg_test_mod_args + {
+       'dependencies': [libpq] + pg_test_mod_args['dependencies'],
+  }

(I'm still green enough with Meson that I had to look that syntax up:
it's how you replace the values of common keys with the values from
the right hand dict[1], which ain't Pythonesque AFAIK.)

For ecpg and pg_regress, where I got stuck last time, I am still a bit
confused about whether it should go here:

-ecpg_inc = include_directories('.')
+ecpg_inc = [libpq_inc, include_directories('.')]

... or perhaps in intermediate ecpgXXX_inc variables or the final
include_directories directives.  In this version I just did that easy
thing and it works for me.  But is it the right place?

Likewise for pg_regress_inc, also used by ECPG and isolation tests:

-pg_regress_inc = include_directories('.')
+pg_regress_inc = [libpq_inc, include_directories('.')]

[1] https://mesonbuild.com/Reference-manual_elementary_dict.html