IMPORTER_REGISTRY is neater and does not do any magical string -> object
conversion. The registry looks more in sync with improvers.
Fixes: aboutcode-org#501

Signed-off-by: Hritik Vijay <hritikxx8@gmail.com>

Etags are meant for transient usage in browsers and are not meant for
any long term usage.
Fixes: aboutcode-org#321

Signed-off-by: Hritik Vijay <hritikxx8@gmail.com>

Signed-off-by: Hritik Vijay <hritikxx8@gmail.com>

The name DataSource and Importer were majorly used interchangeably
although they represent the same concept.
Also, the importers now require a mandatory ``spdx_license_expression``
to run

Signed-off-by: Hritik Vijay <hritikxx8@gmail.com>

Signed-off-by: Hritik Vijay <hritikxx8@gmail.com>

Same for improvers
IMPROVER_REGISTRY -> IMPROVERS_REGISTRY

Signed-off-by: Hritik Vijay <hritikxx8@gmail.com>

Signed-off-by: Hritik Vijay <hritikxx8@gmail.com>

Tests for:
	* manage.py import command
	* manage.py improve command
	* import_runner

Signed-off-by: Hritik Vijay <hritikxx8@gmail.com>

We can use @classmethod and @Property together in python 3.9. Not so in
3.8

Signed-off-by: Hritik Vijay <hritikxx8@gmail.com>

Signed-off-by: Hritik Vijay <hritikxx8@gmail.com>
Co-authored-by: Philippe Ombredanne <pombredanne@gmail.com>