Add the possibility to use a custom goproxy and parse dsc if present

[jjacque]

One way to allow building vendor archive (or even to download dependencies) when private repos are involved is to use an authenticating proxy (ie: https://github.com/goproxy/goproxy). This PR includes some changes / refacto:

• homogenise files / dirs handling using pathlib
• fix various pylint errors:
  • % in logging instead of fstrings
  • create a dedicated function for _create_vendor_archive and _get_gomoddir
  • make functions "private" (no need for docstring)
• add a bit of typing
• use environment variables to define the proxy address (GOPROXY) and the private repos (GONOSUMDB) (see https://go.dev/ref/mod#private-modules)

• also parse dsc when looking for version (in _source_archive_autodetect)

  I integrated https://github.com/openSUSE/obs-service-go_modules/pull/8 in this PR (otherwise the service is constantly failing)

[jjacque]

Splited in 2 PRs (only for the dsc and goproxy features):

• https://github.com/openSUSE/obs-service-go_modules/pull/38
• https://github.com/openSUSE/obs-service-go_modules/pull/39