Improve engine's requires to remove double inclusions

Into the engine.rb template, with 3db5d7b, we started including
the main gem's .rb file in order to have solidus_support available
when the engine was loaded stand-alone (for example when running
`bin/r migration add ...` in an extension). See the PR that the
above commit belongs to for more details.

But that main .rb file was requiring the engine itself so we can
refactor requires in order to have a clean state.

We also changed the `spree/core` require into `solidus_core` since
they are equivalent (`solidus_core` only requires `spree/core`),
see https://github.com/solidusio/solidus/blob/ed88ebc464d1373dc28f4d69cc9fd12c943ddfbe/core/lib/solidus_core.rb .

Co-authored-by: Rainer Dema <rainerdema@users.noreply.github.com>

Author: kennyadsl

lib/solidus_dev_support/templates/extension/lib/%file_name%.rb.tt

@@ -1,8 +1,5 @@
 # frozen_string_literal: true
 
-require 'solidus_core'
-require 'solidus_support'
-
 require '<%=file_name%>/configuration'
 require '<%=file_name%>/version'
 require '<%=file_name%>/engine'

lib/solidus_dev_support/templates/extension/lib/%file_name%/engine.rb.tt

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
-require 'spree/core'
-require '<%= file_name %>'
+require 'solidus_core'
+require 'solidus_support'
 
 module <%= class_name %>
   class Engine < Rails::Engine