tests: Add a test illustrating the issue

Author: voodoos

tests/test-dirs/locate/module-decl-aliases.t

@@ -0,0 +1,61 @@
+  $ cat >main.ml <<EOF
+  > module Bar : sig 
+  >   module Foo : sig
+  >     val v : int
+  >   end
+  > end = struct
+  >   module Foo = struct
+  >     let v = 42
+  >   end
+  > end
+  > module Foo = Bar.Foo
+  > let _ = Foo.v
+  > EOF
+
+  $ cat>other.ml <<EOF
+  > module Foo = Main.Bar.Foo
+  > let _ = Foo.v
+  > EOF
+
+  $ $OCAMLC -c -bin-annot main.ml other.ml
+
+  $ $MERLIN single locate -look-for ml -position 11:10 \
+  > -filename ./main.ml < ./main.ml | jq '.value'
+  {
+    "file": "$TESTCASE_ROOT/main.ml",
+    "pos": {
+      "line": 6,
+      "col": 2
+    }
+  }
+
+FIXME: it would be more useful to traverse the alias and jump to 2:2
+  $ $MERLIN single locate -look-for mli -position 11:10 \
+  > -filename ./main.ml < ./main.ml | jq '.value'
+  {
+    "file": "$TESTCASE_ROOT/main.ml",
+    "pos": {
+      "line": 10,
+      "col": 0
+    }
+  }
+  $ $MERLIN single locate -look-for ml -position 2:10 \
+  > -filename ./other.ml < ./other.ml | jq '.value'
+  {
+    "file": "$TESTCASE_ROOT/main.ml",
+    "pos": {
+      "line": 6,
+      "col": 2
+    }
+  }
+
+FIXME: it would be more useful to traverse the alias and jump to main 2:2
+  $ $MERLIN single locate -look-for mli -position 2:10 \
+  > -filename ./other.ml < ./other.ml | jq '.value'
+  {
+    "file": "$TESTCASE_ROOT/other.ml",
+    "pos": {
+      "line": 1,
+      "col": 0
+    }
+  }