Add merlin locate tests and add build dirs of modules to Merlin so copy# is visible.

[jonahbeckford]

This is a partial follow up to #10178

+bugfix: Adds the build directory of modules to the Merlin source directories so copy# can be followed. Without this modification (merlin.ml:Unprocessed.process) the test

dune/test/blackbox-tests/test-cases/merlin/merlin-locate.t/run.t

Lines 39 to 49 in 2f47fed

	$ cat _build/default/copyroundtrip/copy_of_a.ml
	# 1 "copyroundtrip/a.ml"
	let value = 3
	$ cat ./$LIBNAME/b.ml | ocamlmerlin single locate -look-for ml -position 1:10 -filename b.ml | jq .value
	{
	"file": "$TESTCASE_ROOT/_build/default/copyroundtrip/copy_of_a.ml",
	"pos": {
	"line": 1,
	"col": 0
	}
	}

would be

 $ cat ./$LIBNAME/b.ml | ocamlmerlin single locate -look-for ml -position 1:10 -filename b.ml | jq .value
 "'Copy_of_a' seems to originate from 'Copy_of_a' whose ML file could not be found"

+Adds a test for basic [locate] command

+Adds a "copyroundtrip" test for a [locate] command whose source is from a [copy#] stanza

• Upstream issue: ocamlmerlin should read copy# line header and report the original file not the copied file.

+Adds merlin package to TEST_DEPS so that ocamlmerlin binary can be used in tests

[jonahbeckford]

What is missing: the last step needs to be something like making sure the final file is not In_build_dir.

Today, Merlin does not seem to respect the line header, so it give back a location in In_build_dir. That path should be skipped.

Tomorrow, when Merlin respects the line header and gives back a location in the source tree, that path should be used.

I think that is reasonable logic (although I'm not sure where it goes).