Migrate tool dependencies to go.tools.mod (#14646)

Go 1.14 introduces usage of alternate go.mod files to track
other dependencies which are not related to the main app.
We use this to track all tool dependencies so that everybody uses
the same version of all tools, including CI.

This will prevent version conflicts due to everybody using different
versions of the tools.
And it will not try to upgrade the tool version, every single time,
one runs the tool command.

While here, we also re-generate some filestore mocks which weren't up to date.

Fixes #13088

Co-authored-by: mattermod <mattermod@users.noreply.github.com>

services/filesstore/mocks/FileBackend.go

@@ -1,12 +1,18 @@
 // Code generated by mockery v1.0.0. DO NOT EDIT.
 
-// Regenerate this file using `make files-store-mocks`.
+// Regenerate this file using `make filesstore-mocks`.
 
 package mocks
 
-import io "io"
-import mock "github.com/stretchr/testify/mock"
-import model "github.com/mattermost/mattermost-server/v5/model"
+import (
+	io "io"
+
+	filesstore "github.com/mattermost/mattermost-server/v5/services/filesstore"
+
+	mock "github.com/stretchr/testify/mock"
+
+	model "github.com/mattermost/mattermost-server/v5/model"
+)
 
 // FileBackend is an autogenerated mock type for the FileBackend type
 type FileBackend struct {
@@ -119,15 +125,15 @@ func (_m *FileBackend) ReadFile(path string) ([]byte, *model.AppError) {
 }
 
 // Reader provides a mock function with given fields: path
-func (_m *FileBackend) Reader(path string) (io.ReadCloser, *model.AppError) {
+func (_m *FileBackend) Reader(path string) (filesstore.ReadCloseSeeker, *model.AppError) {
 	ret := _m.Called(path)
 
-	var r0 io.ReadCloser
-	if rf, ok := ret.Get(0).(func(string) io.ReadCloser); ok {
+	var r0 filesstore.ReadCloseSeeker
+	if rf, ok := ret.Get(0).(func(string) filesstore.ReadCloseSeeker); ok {
 		r0 = rf(path)
 	} else {
 		if ret.Get(0) != nil {
-			r0 = ret.Get(0).(io.ReadCloser)
+			r0 = ret.Get(0).(filesstore.ReadCloseSeeker)
 		}
 	}