Refactor where rc files are controlled.

In preparation for https://github.com/bazelbuild/bazel/issues/4502, make OptionProcessor::GetRcFiles contain the logic for both the user bazelrcs and the master bazelrcs.

RELNOTES: None
PiperOrigin-RevId: 193521683
diff --git a/src/main/cpp/workspace_layout.h b/src/main/cpp/workspace_layout.h
index edb2c29..3827b87 100644
--- a/src/main/cpp/workspace_layout.h
+++ b/src/main/cpp/workspace_layout.h
@@ -48,13 +48,9 @@
   // Returns if workspace is a valid build workspace.
   virtual bool InWorkspace(const std::string& workspace) const;
 
-  // Returns the candidate master RC file absolute paths.
-  // All readable files from the result will be used. Empty or nonexistant files
-  // will be ignored. It is ok if no usable candidate exists.
-  virtual std::vector<std::string> FindCandidateBlazercPaths(
+  // Returns the path of the workspace rc file.
+  virtual std::string GetWorkspaceRcPath(
       const std::string& workspace,
-      const std::string& cwd,
-      const std::string& path_to_binary,
       const std::vector<std::string>& startup_args) const;
 
   // Turn a %workspace%-relative import into its true name in the filesystem.