break telegraf registry into separate package

this is for supporting external plugins.

external plugins will depend on a few telegraf interface types, as well
as a common telegraf registry.

this will allow external and internal plugins to both share this package
and make it easier to vendor/version the whole thing semantically, which
will make it easier to keep plugins supported across build and telegraf
versions.

see #1717

plugins/inputs/system/cpu.go

@@ -5,7 +5,7 @@ import (
 	"time"
 
 	"github.com/influxdata/telegraf"
-	"github.com/influxdata/telegraf/plugins/inputs"
+	"github.com/influxdata/telegraf/registry/inputs"
 	"github.com/shirou/gopsutil/cpu"
 )
 

plugins/inputs/system/disk.go

@@ -6,7 +6,7 @@ import (
 	"strings"
 
 	"github.com/influxdata/telegraf"
-	"github.com/influxdata/telegraf/plugins/inputs"
+	"github.com/influxdata/telegraf/registry/inputs"
 )
 
 type DiskStats struct {

plugins/inputs/system/kernel.go

@@ -10,7 +10,7 @@ import (
 	"strconv"
 
 	"github.com/influxdata/telegraf"
-	"github.com/influxdata/telegraf/plugins/inputs"
+	"github.com/influxdata/telegraf/registry/inputs"
 )
 
 // /proc/stat file line prefixes to gather stats on:

plugins/inputs/system/kernel_notlinux.go

@@ -4,7 +4,7 @@ package system
 
 import (
 	"github.com/influxdata/telegraf"
-	"github.com/influxdata/telegraf/plugins/inputs"
+	"github.com/influxdata/telegraf/registry/inputs"
 )
 
 type Kernel struct {

plugins/inputs/system/kernel_vmstat.go

@@ -10,7 +10,7 @@ import (
 	"strconv"
 
 	"github.com/influxdata/telegraf"
-	"github.com/influxdata/telegraf/plugins/inputs"
+	"github.com/influxdata/telegraf/registry/inputs"
 )
 
 type KernelVmstat struct {

plugins/inputs/system/memory.go

@@ -4,7 +4,7 @@ import (
 	"fmt"
 
 	"github.com/influxdata/telegraf"
-	"github.com/influxdata/telegraf/plugins/inputs"
+	"github.com/influxdata/telegraf/registry/inputs"
 )
 
 type MemStats struct {

plugins/inputs/system/net.go

@@ -6,7 +6,7 @@ import (
 	"strings"
 
 	"github.com/influxdata/telegraf"
-	"github.com/influxdata/telegraf/plugins/inputs"
+	"github.com/influxdata/telegraf/registry/inputs"
 )
 
 type NetIOStats struct {

plugins/inputs/system/netstat.go

@@ -5,7 +5,7 @@ import (
 	"syscall"
 
 	"github.com/influxdata/telegraf"
-	"github.com/influxdata/telegraf/plugins/inputs"
+	"github.com/influxdata/telegraf/registry/inputs"
 )
 
 type NetStats struct {

plugins/inputs/system/processes.go

@@ -14,7 +14,7 @@ import (
 	"strconv"
 
 	"github.com/influxdata/telegraf"
-	"github.com/influxdata/telegraf/plugins/inputs"
+	"github.com/influxdata/telegraf/registry/inputs"
 )
 
 type Processes struct {

plugins/inputs/system/system.go

@@ -10,7 +10,7 @@ import (
 	"github.com/shirou/gopsutil/load"
 
 	"github.com/influxdata/telegraf"
-	"github.com/influxdata/telegraf/plugins/inputs"
+	"github.com/influxdata/telegraf/registry/inputs"
 )
 
 type SystemStats struct{}