bug(gatsby-source-strapi): Relations are not picked up if they are named "parent" or "children"

Since "parent" and "children" are reserved keywords in gatsby, this plugin automatically appends "strapi_" to the relation name. Since the relations are now named "strapi_parent" and "strapi_children" the code on this line doesn't work, https://github.com/strapi/gatsby-source-strapi/blob/2a60320b16ae6520617d743158c433d384dbd155/src/download-media-files.js#L117, since the relations on the strapi schema are named just "parent" and "children", but attributeName is "strapi_parent" and "strapi_children".
This problem is related to the other issue I have with relations: https://github.com/strapi/gatsby-source-strapi/issues/308, because without changing the code to `schema.schema.attributes[attributeName.replace('strapi_', '')]` the fix I illustrated in the other issue doesn't work because the fields generated from this plugin and the schema on strapi are different

Start at src/download-media-files.js line 117 and compare the generated relation names with the Strapi schema attributes. Read issue #308 for the related relation behavior, then verify that relations named parent and children are picked up despite their strapi_ prefixes.

Written by the indexing model from the issue text.