Add Elixir Security Live V2 Importer

[michaelehab]

Solves #1933

• Update Elixir Security importer to filter and process advisories relevant to the purl passed in the constructor
• Update Elixir Security v2 importer to filter and process advisories relevant to the purl passed in the constructor
• Update Elixir Security importer tests to include testing package-first mode

[michaelehab]

@TG1999 I merged the recent fixes here and did another functional test for the v2 importer. I'd appreciate your feedback and review, thanks.