Data Migrator does not work with self referential many to many entity relationships

**Describe the bug**
If an out of the box N:N relationship is set up between a table and itself, the data migrator cannot export it, as it assumes the primary key for records will always be "schema name" + id, but with self referential N:Ns the primary key on the link table is is "schema name" +
"schema name" + id, with the guids of the two records to be linked stored as "schema name" + idone and "schema name" + idtwo.

**To Reproduce**
Create an OOB self referential many to many relationship between a table and itself.
Add this relationship to the schema in CDS Data Migrator.
Attempt to export data using the schema.

**Expected behavior**
CDS data migrator should export the records with the relationship and model it correctly in JSON.

**Additional context**
This is something that the base configuration migration tool handles so we have been using that just for this one table relationship and CDS Data Migrator for everything else. I assume these parameters in the CM schema are used to manage this scenario: `manyToMany="true" isreflexive="true" `

Start by reproducing the export with an out-of-the-box self-referential many-to-many relationship in CDS Data Migrator, then compare its behavior with the base configuration migration tool. Inspect how the export handles the schema parameters `manyToMany="true"` and `isreflexive="true"`, and verify that the linked records and relationship are represented correctly in JSON.

Written by the indexing model from the issue text.