Update Error Prone -XepExcludedPaths flag to be Windows-compatible (#927)

Prior to these changes the provided pattern would never match on
Windows, as Error Prone matches against file URIs, which in practice
will always contain forward slashes, even on Windows.

pom.xml

@@ -1741,7 +1741,13 @@
                                     avoid that, so we simply tell Error Prone
                                     not to warn about generated code. -->
                                     -XepDisableWarningsInGeneratedCode
-                                    -XepExcludedPaths:\Q${project.build.directory}${file.separator}\E.*
+                                    <!-- XXX: Ideally we'd match against
+                                    `\Q${project.build.directory}${file.separator}\E.*`,
+                                    but that approach doesn't work on Windows,
+                                    as Error Prone matches against a path that
+                                    always contains forward-slash path
+                                    separators. -->
+                                    -XepExcludedPaths:(?!.*/src/[^/]+/java/.*).*
                                     <!-- We don't target Android. -->
                                     -Xep:AndroidJdkLibsChecker:OFF
                                     <!-- XXX: Enable this once we open-source