Fixed duplicate exportpath labels.

Fixed duplicate exportpath labels caused by the integrattion
of export_unity_package in I6fbb9da338ee2c29782226e95c17c2e485d191e7.
In addition, this change export paths to be calculated relative to
the plugin install location (i.e in Assets/ or Packages/) and handle
export paths that have Assets/ as the root of their path which is
typical for .unitypackage plugins.

Change-Id: I4d30d0a6f1f106a60ca871ebbfd13ab1966f1bf9

Author: Stewart Miles

build.gradle

@@ -644,7 +644,7 @@ File copyAssetMetadataFile(File sourceFile, File targetFile) {
       } else if (exportPathMatch.matches()) {
         // Update the export path of the asset.
         line = sprintf("%s%s", exportPathMatch.group(1),
-                       targetFile.path.replaceFirst(/.*\/Assets\//, "Assets/"))
+                       targetFile.path.replaceFirst(/.*\/Assets\//, ""))
         line = line.substring(0, line.length() -
                               project.ext.unityMetadataExtension.length())
       }