feat: Add new data structures for an abstract representation of projects/findings

[tarkatronic]

Fixes #47

This is my first stab at a proposed data structure for data sources to represent their findings, grouped by project. I believe it would end up being used something like this (very much not valid code):

// scan.go
// ...
projects := NewProjectCollection()
for _, ds := range datasources {
    ds.CollectFindings(projects)
}

// github.go (GitHub datasource)
// ... call the GraphQL API ...
for _, repo := range result.repositories {
    project := projects.GetProject(repo.Name)
    for _, vuln := range repo.VulnerabilityAlerts {
        finding := project.GetFinding(vuln.SecurityAdvisory.Identifiers)
        if finding.Description == "" {
            finding.Description = vuln.SecurityAdvisory.Summary
        }
    }
}

[codecov]

Codecov Report

Merging #75 (934a1b4) into main (bfce6fe) will increase coverage by 4.15%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #75      +/-   ##
==========================================
+ Coverage   52.06%   56.22%   +4.15%     
==========================================
  Files          13       14       +1     
  Lines         580      635      +55     
==========================================
+ Hits          302      357      +55     
  Misses        272      272              
  Partials        6        6              

Flag	Coverage Δ
unittests	56.22% <100.00%> (+4.15%)	⬆️

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Files Changed	Coverage Δ
querying/project.go	100.00% <100.00%> (ø)

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