[xbuild] Handle a repeat build of a target, when building through ..

[xbuild] Handle a repeat build of a target, when building through ..

.. `Project.Build(..)`. For example, in:

```
	<MSBuild Projects="bar.proj" Targets="foo;foo" />
```

The second invocation of `foo`, in the *same* `Project.Build(..)` call,
would get skipped, but BuildTarget assumed that no previous `outputs`
entry existed in the `targetOutputs` dictionary.

System.Exception: MSBuild operation failed ---> System.ArgumentException: Item has already been added. Key in dictionary: 'foo'  Key being added: 'foo'
  at System.Collections.Hashtable.Insert (System.Object key, System.Object nvalue, System.Boolean add) [0x00200] in <94fd79a3b7144c54b4cb162b50fc7761>:0
  at System.Collections.Hashtable.Add (System.Object key, System.Object value) [0x00000] in <94fd79a3b7144c54b4cb162b50fc7761>:0
  at Microsoft.Build.BuildEngine.Project.BuildTarget (System.String target_name, System.Collections.IDictionary targetOutputs) [0x0007e] in <b1524340ac7e4c06b22fac1b2db62c08>:0
  at Microsoft.Build.BuildEngine.Project.BuildInternal (System.String[] targetNames, System.Collections.IDictionary targetOutputs, Microsoft.Build.BuildEngine.BuildSettings buildFlags) [0x00134] in <b1524340ac7e4c06b22fac1b2db62c08>:0
  at Microsoft.Build.BuildEngine.Project.Build (System.String[] targetNames, System.Collections.IDictionary targetOutputs, Microsoft.Build.BuildEngine.BuildSettings buildFlags) [0x0004b] in <b1524340ac7e4c06b22fac1b2db62c08>:0

Fixes: https://bugzilla.xamarin.com/show_bug.cgi?id=44549