feat: expose sbom generation functionality

[nscuro]

This WIP PR aims to expose the core functionality of cyclonedx-gomod, so it can be used by other applications.
"Core functionality" referring to the three SBOM generation commands app, bin and mod.

Addresses #99, #108.

Tasks

• Define public API
• Migrate app logic
• Migrate bin logic
• Migrate mod logic
• Ensure that there's Godoc for everything public
• Figure out how to deal with internal functionality that uses the global zerolog logger.
  For a clean experience, everything should use the loggers passed to the Generator implementation's constructors,
  but that will require quite a bit of refactoring.
  • Refactored internal functions that log to accept a logger parameter. Feels dirty, but does the job for now.
• Add a disclaimer somewhere that cyclonedx-gomod will not be investing in staying compatible with older Go versions.
  For example, we may adapt Go 1.18 features shortly after its GA.
• Ensure tests continue to work across multiple platforms
  I don't want to continue dragging reproducible flags through the code, but it's also quite challenging to emulate static environments (wrt Go version, GOOS, GOARCH etc.). Maybe stripping some fields of the BOM before comparing it to its snapshot is the way to go here...