Create public models for telegraf metrics, accumlator, plugins

This will basically make the root directory a place for storing the
major telegraf interfaces, which will make telegraf's godoc looks quite
a bit nicer. And make it easier for contributors to lookup the few data
types that they actually care about.

closes #564

internal/models/filter.go

@@ -1,4 +1,4 @@
-package models
+package internal_models
 
 import (
 	"strings"

internal/models/filter_test.go

@@ -1,4 +1,4 @@
-package models
+package internal_models
 
 import (
 	"testing"

internal/models/running_input.go

@@ -1,14 +1,14 @@
-package models
+package internal_models
 
 import (
 	"time"
 
-	"github.com/influxdata/telegraf/plugins/inputs"
+	"github.com/influxdata/telegraf"
 )
 
 type RunningInput struct {
 	Name   string
-	Input  inputs.Input
+	Input  telegraf.Input
 	Config *InputConfig
 }
 

internal/models/running_output.go

@@ -1,10 +1,10 @@
-package models
+package internal_models
 
 import (
 	"log"
 	"time"
 
-	"github.com/influxdata/telegraf/plugins/outputs"
+	"github.com/influxdata/telegraf"
 
 	"github.com/influxdata/influxdb/client/v2"
 )
@@ -13,7 +13,7 @@ const DEFAULT_POINT_BUFFER_LIMIT = 10000
 
 type RunningOutput struct {
 	Name             string
-	Output           outputs.Output
+	Output           telegraf.Output
 	Config           *OutputConfig
 	Quiet            bool
 	PointBufferLimit int
@@ -24,7 +24,7 @@ type RunningOutput struct {
 
 func NewRunningOutput(
 	name string,
-	output outputs.Output,
+	output telegraf.Output,
 	conf *OutputConfig,
 ) *RunningOutput {
 	ro := &RunningOutput{