Fix vlib-default-impl for dune-package

The library that was used for default-implementation didn't exist
previously and our error checking was lax.

Signed-off-by: Rudi Grinberg <rudi.grinberg@gmail.com>

test/blackbox-tests/test-cases/vlib-default-impl/dune-package/a-default/dune

@@ -0,0 +1,4 @@
+(library
+ (name a_default)
+ (public_name a.default-impl)
+ (implements a))

test/blackbox-tests/test-cases/vlib-default-impl/dune-package/dune

@@ -1,7 +1,7 @@
 (library
  (public_name a)
  (virtual_modules x)
- (default_implementation a-default))
+ (default_implementation a_default))
 
 (rule
  (with-stdout-to

test/blackbox-tests/test-cases/vlib-default-impl/run.t

@@ -33,15 +33,55 @@ Check that ambiguity is handled correctly.
 Check that default implementation data is installed in the dune package file.
   $ dune build --root dune-package
   Entering directory 'dune-package'
-  File "dune", line 4, characters 25-34:
-  4 |  (default_implementation a-default))
-                               ^^^^^^^^^
-  Error: Library "a-default" not found.
-  Hint: try: dune external-lib-deps --missing --root dune-package @@default
-  [1]
   $ cat dune-package/_build/install/default/lib/a/dune-package
-  cat: dune-package/_build/install/default/lib/a/dune-package: No such file or directory
-  [1]
+  (lang dune 2.0)
+  (name a)
+  (library
+   (name a)
+   (kind normal)
+   (virtual)
+   (foreign_archives (native a$ext_lib))
+   (default_implementation a.default-impl)
+   (main_module_name A)
+   (modes byte native)
+   (modules
+    (wrapped
+     (main_module_name A)
+     (modules
+      ((name X) (obj_name a__X) (visibility public) (kind virtual) (intf)))
+     (alias_module
+      (name A)
+      (obj_name a)
+      (visibility public)
+      (kind alias)
+      (impl))
+     (wrapped true))))
+  (library
+   (name a.default-impl)
+   (kind normal)
+   (archives
+    (byte default-impl/a_default.cma)
+    (native default-impl/a_default.cmxa))
+   (plugins
+    (byte default-impl/a_default.cma)
+    (native default-impl/a_default.cmxs))
+   (foreign_archives (native default-impl/a_default$ext_lib))
+   (requires a)
+   (implements a)
+   (main_module_name A)
+   (modes byte native)
+   (modules
+    (wrapped
+     (main_module_name A)
+     (modules
+      ((name X) (obj_name a__X) (visibility public) (kind impl_vmodule) (impl)))
+     (alias_module
+      (name A__a_default__)
+      (obj_name a__a_default__)
+      (visibility public)
+      (kind alias)
+      (impl))
+     (wrapped true))))
 
 Test default implementation for an external library