EPUB export: pick up XMP metadata from 'metadata.xmp'

This used to be <filename>.xmp, but that's inconsistent with how cover
image is not <filename>.jpg but cover.jpg (or .png, etc).

Also it was redundant, since these files are in the media directory, for
which the default is the <filename> directory.

Change-Id: Iadfebf2f9c336f443537759b1e58409a93e1c51d
Reviewed-on: https://gerrit.libreoffice.org/49604Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>

writerperfect/source/writer/exp/xmlimp.cxx

@@ -183,8 +183,7 @@ void FindXMPMetadata(const uno::Reference<uno::XComponentContext> &xContext, con
         return;
 
     OUString aMediaDir = FindMediaDir(rDocumentBaseURL, rFilterData);
-    INetURLObject aDocumentBaseURL(rDocumentBaseURL);
-    OUString aURL = aMediaDir + aDocumentBaseURL.GetBase() + ".xmp";
+    OUString aURL = aMediaDir + "metadata.xmp";
     SvFileStream aStream(aURL, StreamMode::READ);
     if (!aStream.IsOpen())
         return;