Feature: map the current Dependencydiff package naming convention (GitHub) to OSV

[aidenwang9867]

Is your feature request related to a problem? Please describe.
In v0 of the API/CLI Dependencydiff (PR #2046/PR #2077), the formats (convention) of dependency fields like the package ecosystem (GH uses pypi for python pkgs whereas OSV uses pip) follow the GitHub naming convention. Examples:

[
  {
    "changeType": "added",
    "packageUrl": "pkg:golang/github.com/google/go-github/v35@35.2.0",
    "sourceRepository": null,
    "manifestPath": "go.mod",
    "ecosystem": "gomod",
    "version": "35.2.0",
    "name": "github.com/google/go-github/v35",
  },
  {
    "changeType": "updated",
    "packageUrl": "pkg:golang/gocloud.dev@0.23.0",
    "sourceRepository": "https://github.com/google/go-cloud",
    "manifestPath": "go.mod",
    "ecosystem": "gomod",
    "version": "0.23.0",
    "name": "gocloud.dev",
  },
]

Describe the solution you'd like
By @laurentsimon: since we may use a different Dependency-diff source API in the future, it would be nice not to get stuck using GH naming convention.

I can do the ecosystem mapping using this list (near The defined ecosystems are:), by defining a static map in the scorecard code to convert the GH ecosystems to the OSV ones. I'll use a follow-up PR to do this mapping very soon.