De-duplicate BuildTarget.sources

If the same source is provided by multiple dependencies it was added
multiple times, as `added_sources` was only guarding against duplicates
within the same source list. This was not a problem with ninja, but it
triggers multiple sanity checks within xcode backend while attempting to
create multiple ids for the same file.

Rename `added_sources` to `seen_sources` as per reviewers request

mesonbuild/build.py

@@ -762,6 +762,8 @@ class BuildTarget(Target):
         self.pch: T.Dict[str, T.List[str]] = {}
         self.extra_args: T.DefaultDict[str, T.List[str]] = kwargs.get('language_args', defaultdict(list))
         self.sources: T.List[File] = []
+        # If the same source is defined multiple times, use it only once.
+        self.seen_sources: T.Set[File] = set()
         self.generated: T.List['GeneratedTypes'] = []
         self.extra_files: T.List[File] = []
         self.d_features: DFeatures = {
@@ -884,12 +886,11 @@ class BuildTarget(Target):
            (static as they are only regenerated if meson itself is regenerated)
         3. Sources files generated by another target or a Generator (generated)
         """
-        added_sources: T.Set[File] = set() # If the same source is defined multiple times, use it only once.
         for s in sources:
             if isinstance(s, File):
-                if s not in added_sources:
+                if s not in self.seen_sources:
                     self.sources.append(s)
-                    added_sources.add(s)
+                    self.seen_sources.add(s)
             elif isinstance(s, (CustomTarget, CustomTargetIndex, GeneratedList)):
                 self.generated.append(s)