Migrate Source YAML Fileset

Allows a set of YAML files to be used as a source for migrations. Each file represents a row for migration.

In my_migrate_module/migrations/pages.yml:

id: pages
source:
  plugin: yaml_fileset
  path: 'modules/custom/my_migrate_module/content/pages'
  ids:
    id:
      type: string
process:
  title: title
  body/value: body
  body/format:
    plugin: default_value
    default_value: plain_text
    source: body_format
destination:
  plugin: 'entity:node'
  default_bundle: page