Split spec files programmatically

Instead of shelling out to rspec, we now split spec files into
individual examples programmatically, using the RSpec Core API. This is
arguably faster and more robust.

Closes #6

lib/rspecq/worker.rb

@@ -168,33 +168,22 @@ def try_publish_queue!(queue)
     # falling back to scheduling them as whole files. Their errors will be
     # reported in the normal flow when they're eventually picked up by a worker.
     def files_to_example_ids(files)
-      cmd = "DISABLE_SPRING=1 bundle exec rspec --dry-run --format json #{files.join(' ')} 2>&1"
-      out = `#{cmd}`
-      cmd_result = $?
+      out = StringIO.new
 
-      if !cmd_result.success?
-        rspec_output = begin
-                         JSON.parse(out)
-                       rescue JSON::ParserError
-                         out
-                       end
-
-        log_event(
-          "Failed to split slow files, falling back to regular scheduling",
-          "error",
-          rspec_output: rspec_output,
-          cmd_result: cmd_result.inspect,
-        )
-
-        pp rspec_output
-
-        return files
-      end
-
-      JSON.parse(out)["examples"].map { |e| e["id"] }
+      RSpec.configuration.add_formatter(RSpec::Core::Formatters::JsonFormatter.new(out))
+      RSpec.configuration.files_or_directories_to_run = files_or_dirs_to_run
+      RSpec.configuration.dry_run = true
+
+      opts = RSpec::Core::ConfigurationOptions.new(files)
+      result = RSpec::Core::Runner.new(opts).run($stderr, $stdout)
+      return files if result != 0
+      JSON.parse(out.string)["examples"].map { |e| e["id"] }
+    ensure
+      RSpec.configuration.dry_run = false
+      # clear formatters
+      reset_rspec_state!
     end
 
-
     private
 
     def reset_rspec_state!