Fix concretization bugs with virtuals and deptypes.

1. Fix #2807: Can't depend on virtual and non-virtual package

- This is tested by test_my_dep_depends_on_provider_of_my_virtual_dep in
  the concretize.py test.

- This was actually working in the test suite, but it depended on the
  order the dependencies were resolved in. Resolving non-virtual then
  virtual worked, but virtual, then non-virtual did not.

- Problem was that an unnecessary copy was made of a spec that already
  had some dependencies set up, and the copy lost half of some of the
  dependency relationships.  This caused the "can'd depend on X twice
  error".

- Fix by eliminating unnecessary copy and ensuring that dep parameter of
  _merge_dependency is always safe to own -- i.e. it's a defensive copy
  from somewhere else.

2. Fix bug and simplify concretization of deptypes.

- deptypes weren't being accumulated; they were being set on each
  DependencySpec. This could cause concretization to get into an infinite
  loop.

- Fixed by accumulating deptypes in DependencySpec.update_deptypes()

- Also simplified deptype normalization logic: deptypes are now merged in
  constrain() like everything else -- there is no need to merge them
  specially or to look at dpeendents in _merge_dependency().

- Add some docstrings to deptype tests.