feat(output): add a unified output result (#1397)

Refactored the
[HTMLResult](https://github.com/google/osv-scanner/blob/9ea8aa57b45890a288a138b600900679ba6c98d1/internal/output/html.go#L19)
into a unified structure that can be reused by other output formats.
This new structure presents results hierarchically, grouping
vulnerabilities by ecosystem, source, and package. Compare to the
original vulnerabilityResult, it contains more information, such as
vulnerability count (called/uncalled/fixable/unfixable/severity),
package/vulnerability fix information, etc.

Follow-up PRs:
- Merge this into the v2 branch (via
#1396)
- Leverage this structure to generate a table package-view output.
- Update html.go and HTML templates to use the OutputResult.