Adapt rust importer to new advisory format

[sbs2001]

Rust advisory db has moved on from using plain toml to using markdown + toml 'front matter' .
This PR enables the current rust importer to import data from the new format.

[sbs2001]

Fixes #280

[pombredanne]

Thanks!
See some comments inline. Also try to avoid the big test files. Testting the opening of a zip file is rather trivial. One the other hand having a few test markdown as text files would make sense.

+515 KB vulnerabilities/tests/test_data/advisory-db.zip
+365 KB (170%) vulnerabilities/tests/test_data/rust-advisory-db.zip

[sbs2001]

@pombredanne about zip files :

Those are compressed git repositories. I tried trimming these but I was getting mysterious bugs and just gave up on that. We have rust-advisory-db.zip and advisory-db.zip instead of a single rust-advisory-db.zip because the tests in test_data_source.py require the old kind of rust-advisory-db.

[pombredanne]

re:

Those are compressed git repositories. I tried trimming these but I was getting mysterious bugs and just gave up on that.

IMHO we should be able to create smaller zip with fewer files. How did you do this?

We have rust-advisory-db.zip and advisory-db.zip instead of a single rust-advisory-db.zip because the tests in test_data_source.py require the old kind of rust-advisory-db.

Then may be renaming these to have a clearer name would help.

[pombredanne]

nit: sort the imports

[pombredanne]

I made a small suggestion for your consideration. LGTM otherwise!
Thank you