Extract repository information from BOM

[ribbybibby]

Fixes #5

Infer repositories from Go package names and extract them from the external references in a CycloneDX SBOM.

This ended up being quite a big piece of work, mostly because I wanted to refactor things so adding new sources of repository information will be more straightforward in future.

1. Create a repositories.Mapper interface that retrieves repositories for a package. This enables multiple implementations that use different methods to find repositories.
2. Implement a repositories.Mapper that infers the repository from the Go package names (if it can)
3. Tweak the db interfaces to add a db.RepositoryReader that only implements the GetRepositories method
4. Implement a repositories.Mapper that gets repositories from the tally database with the db.RepositoryReader interface
5. Refactor the bom parsing to produce a 'BOM' interface that can be used to get repositories from the BOM. The repositories are parsed out of the external references with some custom logic to handle the myriad of different formats a github URL can appear in.
6. Implement a repositories.Mapper that gets repositories from the new BOM interface
7. Add tests for all this craziness