Modify Rust importer to support package-first mode

[michaelehab]

Solves #1911

• Update Rust importer to only load and process advisories relevant to the purl passed in the constructor
• Update Rust importer tests to include testing the package-first mode

[TG1999]

Suggested change

	if self.purl:
	if self.purl and self.purl.type!="cargo"

[TG1999]

We are not running Rust importers either in V1 nor in V2 as of now, I think we need to first make importer functional before doing this.

[pombredanne]

@michaelehab can you join the community calls and chat rooms so we can discuss this?

[TG1999]

@michaelehab it will be helpful for us mentors and community members to design and help you if you can regularly join us in VCIO community calls that happens on Tuesday 8:00 AM PST https://meet.jit.si/VulnerableCode

[michaelehab]

@TG1999 @pombredanne Sure, I will start joining this call too regularly so we can discuss this more.
Regarding this PR, I worked on it to show how the Rust importer can be potentially supported like other importers. I wanted to mark it as a draft PR, but I'm not sure I have the access to do this.