Add getTemplateByName method to TemplateStorage

Very much in favor of this one, I wonder though if it should contain an error rather then returning NULL on missing templates.

Time to push this into the RTBC stack. Now with a test!! I have been running this patch for most of my migrations in the last few days and it has been thoroughly tested.

This is a blocker for #2530030: Create the migrate builder plugin type.

1. +++ b/core/modules/migrate/src/MigrateTemplateStorage.php
   @@ -63,6 +63,22 @@ public function findTemplatesByTag($tag) {
   +   * @return null|array
   

   NULL should be capitals

2. +++ b/core/modules/migrate/src/MigrateTemplateStorage.php
   @@ -63,6 +63,22 @@ public function findTemplatesByTag($tag) {
   +    return isset($templates[$name]) ?
   +      $templates[$name] :
   +      NULL;
   

   Our coding standards has this on one line.

3. +++ b/core/modules/migrate/src/Tests/TemplateTest.php
   @@ -49,4 +47,29 @@ public function testTemplates() {
   +  public function testGetTemplateByName() {
   

   Missing function comment.

The previous patch was just minor style fixes, so back to RTBC.

@prajaankit, what is your patch adding or removing? All I see are formatting changes. Please post some explanation, and an interdiff.

Patch is double-plus good, and blocks (at least) #2530030: Create the migrate builder plugin type and #2522660: Templates Broke migrate-manifest. Should we tag this "Migrate critical"?

In a word: yes.

Committed 92965c5 and pushed to 8.0.x. Thanks!