[GSoC 2021] Adapt Schema Editors to operate from model states instead of fake rendered models

Populated the central registry.(PR-14587)

• Created a NamedTuple List to store all relations in a variable named as related_fields.
• Added registry population logic and code in django.db.migraztions.state.ProjectState.add_model.
• Added registry population logic and code in django.db.migraztions.state.ProjectState.remove_model
• Added registry population logic and code in django.db.migraztions.state.ProjectState.add_field
• Added registry population logic and code in django.db.migraztions.state.ProjectState.rename_model
• Added registry population logic and code in django.db.migraztions.state.ProjectState.rename_field

• The pull request got reviewed.
• Requested changes are mentioned here
  

• Worked on improving the complexity of newly introduced registry.
• The conclusion is discussed here

• Simon suggested that a registry is already present and we may use that as the problem of complexity of old registry still persists.
• The suggestion can be seen here

Added the Following changes and made a commit.

• Redesigned the whole central registry and relations mapping.
• Added the changes in the commit c3d1625.

• The pull request got reviewed.
• Requested changes are mentioned here
  

Added the Following changes and made a commit.

• Removed the automatic resolution of fields and relation for relations registry.
• Removed unnecessary work done to calculate lower.
• Used @cached_property decorator to invalidate the cache logics.
• Added Tests.
• Added the changes in the commit f8e585a.

• The pull request got reviewed.
• Requested changes are mentioned here
  

Added the Following changes and made a commit.

• created resolve_model_field_relations to encapsulate the logic of adding a field relation to the registry.
• used resolve_model_field_relations in populate_relation_from_model_state.
• Removed the @cached_property decorator.
• Changed relations to a property and added a new internal variable named as _relations.
• Removed redundant tests.
• Changed the tests location from tests.migrations.test_operation to tests.migrations.test_state.
• Used ProjectState proxied functions to test the changes in spite of migration operations.
• Added the changes in the commit b1cf0c3.
• Added test related changes in the commit e9eaf87.