Rework patch to work without accident

The previous iteration of the patch worked somewhat by accident. After
digging in and figuring out why the two dep sources are the way they
are, the patch is now properly working with a well-documented
explanatiion inline.

src/rebar_prv_upgrade.erl

@@ -45,13 +45,29 @@ init(State) ->
 do(State) ->
     {Args, _} = rebar_state:command_parsed_args(State),
     Locks = rebar_state:get(State, {locks, default}, []),
-    %% grab the parsed deps, and then add in the deps found in each of the
-    %% umbrella apps' configs (which are in {deps, default} when added by
-    %% `rebar_app_discover'). This later set however includes locks, so
-    %% we remove them to get a full picture.
+    %% We have 3 sources of dependencies to upgrade from:
+    %% 1. the top-level rebar.config (in `deps', dep name is an atom)
+    %% 2. the app-level rebar.config in umbrella apps (in `{deps, default}',
+    %%    where the dep name is an atom)
+    %% 3. the formatted sources for all after app-parsing (in `{deps, default}',
+    %%    where the reprocessed app name is a binary)
+    %%
+    %% The gotcha with these is that the selection of dependencies with a
+    %% binary name (those that are stable and usable internally) is done with
+    %% in the profile deps only, but while accounting for locks.
+    %% Because our job here is to unlock those that have changed, we must
+    %% instead use the atom-based names, both in `deps' and `{deps, default}',
+    %% as those are the dependencies that may have changed but have been
+    %% disregarded by locks.
+    %%
+    %% As such, the working set of dependencies is the addition of
+    %% `deps' and `{deps, default}' entries with an atom name, as those
+    %% disregard locks and parsed values post-selection altogether.
+    %% Packages without versions can of course be a single atom.
     TopDeps = rebar_state:get(State, deps, []),
     ProfileDeps = rebar_state:get(State, {deps, default}, []),
-    Deps = TopDeps ++ ((ProfileDeps -- TopDeps) -- Locks), % top deps > profile deps
+    Deps = [Dep || Dep <- TopDeps ++ ProfileDeps, % TopDeps > ProfileDeps
+                   is_atom(Dep) orelse is_atom(element(1, Dep))],
     Names = parse_names(ec_cnv:to_binary(proplists:get_value(package, Args, <<"">>)), Locks),
     DepsDict = deps_dict(rebar_state:all_deps(State)),
     case prepare_locks(Names, Deps, Locks, [], DepsDict) of