-I should always win over gems

If there were gems in the "unresolved" list, then the files in that gem
would be preferred over files that included in -I.  For example, if you
had a depdedncy tree that looks like this:

```dot
digraph g {
  a -> b [label="version: = 1"];
  b -> c [label="version: > 0"];
}
```

and the installed gems were like this:

a = 1,
b = 1,
c = 1,
c = 2,

When the user requires 'a', then 'b' will be immediately activated
because there is only one decision to make.  However, 'c' will be added
to the `unresloved_deps` list.  Having anything in that list will cause
Rubygems [to skip checking if Ruby's `require` can handle the file](https://github.com/rubygems/rubygems/blob/07f1a83fe095e2b9b09e2f6b9661aed326b2de6a/lib/rubygems/core_ext/kernel_require.rb#L52-55)
If the user provides any paths to `-I`
that contain files that are also in an activated gem (like `b` in this
case), then Rubygems will prefer the file from the gem rather than the
file provided by -I.

This commit fixes it so that -I will win over gems.  e2935c8 introduced
the incorrect behavior.

lib/rubygems/core_ext/kernel_require.rb

@@ -66,7 +66,7 @@ def require path
 
     begin
       RUBYGEMS_ACTIVATION_MONITOR.exit
-      return gem_original_require(spec.to_fullpath(path) || path)
+      return gem_original_require(path)
     end if spec
 
     # Attempt to find +path+ in any unresolved gems...

test/rubygems/test_require.rb

@@ -49,6 +49,36 @@ def append_latch spec
     end
   end
 
+  # Providing -I on the commandline should always beat gems
+  def test_dash_i_beats_gems
+    a1 = new_spec "a", "1", {"b" => "= 1"}, "lib/test_gem_require_a.rb"
+    b1 = new_spec "b", "1", {"c" => "> 0"}, "lib/b/c.rb"
+    c1 = new_spec "c", "1", nil, "lib/c/c.rb"
+    c2 = new_spec "c", "2", nil, "lib/c/c.rb"
+
+    install_specs c1, c2, b1, a1
+
+    dir = Dir.mktmpdir
+    dash_i_arg = File.join Dir.mktmpdir, 'lib'
+
+    c_rb = File.join dash_i_arg, 'b', 'c.rb'
+
+    FileUtils.mkdir_p File.dirname c_rb
+    File.open(c_rb, 'w') { |f| f.write "class Object; HELLO = 'world' end" }
+
+    lp = $LOAD_PATH.dup
+
+    # Pretend to provide a commandline argument that overrides a file in gem b
+    $LOAD_PATH.unshift dash_i_arg
+
+    assert_require 'test_gem_require_a'
+    assert_require 'b/c' # this should be required from -I
+    assert_equal "world", ::Object::HELLO
+  ensure
+    $LOAD_PATH.replace lp
+    Object.send :remove_const, :HELLO if Object.const_defined? :HELLO
+  end
+
   def test_concurrent_require
     Object.const_set :FILE_ENTERED_LATCH, Latch.new(2)
     Object.const_set :FILE_EXIT_LATCH, Latch.new(1)