Rename custom_block.module to block_custom.module

Follow-up to: #1871772: Convert custom blocks to content entities

Problem

• custom* module names are the informal standard namespace for custom developed, site-specific modules.
• I have at least a dozen of sites that have a custom_block.module already (among other custom* modules).

Proposed solution

1. Rename custom_block.module to block_custom.module.
2. Move the module directly into /core/modules/
3. Add a block_custom.module component for issues to d.o.