interpreter: Always flatten when unholding arrays

Otherwise we might end up with wrapper holders in the Build object and
pickling will then fail, defeating the purpose of the holder objects.

Closes https://github.com/mesonbuild/meson/issues/2211

mesonbuild/mesonlib.py

@@ -707,6 +707,7 @@ def windows_proof_rmtree(f):
 
 def unholder_array(entries):
     result = []
+    entries = flatten(entries)
     for e in entries:
         if hasattr(e, 'held_object'):
             e = e.held_object

test cases/common/87 declare dep/entity/meson.build

@@ -1,6 +1,6 @@
 entity_lib = static_library('entity', 'entity1.c')
 
-entity_dep = declare_dependency(link_with : entity_lib,
+entity_dep = declare_dependency(link_with : [[entity_lib]],
   include_directories : include_directories('.'),
   sources : 'entity2.c',
   compile_args : ['-DUSING_ENT=1'],