Refactor the gittag plugin + fix sources of gittag

[dduportal]

This PR comes from the work in progress in #412 and refactors the gittag plugin into its own package.

While working on this, 2 bugs where found on this plugin and reported (#467 #468). These 2 bugs are now corrected

Fix #467
Fix #469

Test

To test this pull request, you can run the following commands:

go build -o ./dist/updatecli && ./dist/updatecli diff --config examples/updatecli.generic/gitTag.yaml

should error only on the 2 targets (expected) with:

ERROR: Git push tag error: authentication required
ERROR: Something went wrong in target "git" :
ERROR: authentication required

Additional Information

• Please note that I did work on the conditions/targets, neither on unit tests for this one
  • factorizing code on condition and target between "normal" and "SCM" methods
  • Using unit test with mock
• The fix embeded on this PR would allow the scenario "get latest stable nginx" (it wasn't possible before):

title: Bump nginx:stable docker image version

scms:
  default:
    kind: github
    spec:
      user: "{{ .github.user }}"
      email: "{{ .github.email }}"
      owner: "{{ .github.owner }}"
      repository: "{{ .github.repository }}"
      token: "{{ requiredEnv .github.token }}"
      username: "{{ .github.username }}"
      branch: "{{ .github.branch }}"
  nginx-gh-mirror:
    kind: git
    spec:
      url: "https://github.com/nginx/nginx.git"
      branch: "master"

sources:
  latestRelease:
    name: Get latest stable version of nginx
    kind: gitTag
    scmID: nginx-gh-mirror
    spec:
      versionFilter:
        kind: regex
        ## Nginx stable version have the minor digit as an even number
        pattern: 'release-(\d+)\.(\d*[0|2|4|6|8])\.(\d+)'
    transformers:
      - trimPrefix: "release-"
      - addSuffix: "-alpine"

[dduportal]

Special addon for you @olblak : after merging #419, I've added a e2e test for gittag sources :)

[lemeurherve]

Small nipticks, LGTM otherwise!

[dduportal]

Thanks for the review folks! I've enabled auto-merge (with squash).

I've taken in account almost all feedbacks (except 2: I've added comments).

As soon as you approve, this will be merged : subsequent comment will need ...a subsequent PR ;)