Feature: repo/file interface for Scorecard library

[azeemshaikh38]

Is your feature request related to a problem? Please describe.
Scorecard today is limited to run on GitHub repositories only. We should expand this support to handle local files and other VCS like GitLab, BitBucket etc.

Describe the solution you'd like
Create an interface RepoClient:

type RepoClient interface {
  Commits()
  File()
  ...
}

We need to look into our current code and sketch out all the required methods needed in this interface. Update our code bit-by-bit to use this interface instead of directly relying on GitHub API.

Under the hood use GitHubClient/LocalClient/GitLabClient which implements this interface and can fit clients needs.

[azeemshaikh38]

@inferno-chromium @oliverchang @laurentsimon @naveensrinivasan thoughts?

[oliverchang]

+1. Sounds great!

[inferno-chromium]

+1, this would be awesome, and we definitely need localclient support. some checks might not be applicable when provided a local repo, but we need to add some skip state for those (instead of fail).

[naveensrinivasan]

+1, this would be awesome, and we definitely need localclient support. some checks might not be applicable when provided a local repo, but we need to add some skip state for those (instead of fail).

+1. Great!!

[laurentsimon]

+1