If authFile is missing, try to use the user's default .netrc file (~/.netrc)

[grepwood]

I have come across this problem at work and I needed to be able to generate a workspace.bzl that includes some kind of way to read my .netrc file without specifying a path to it in the authFile parameter, because this will never ever be the same constant path on developer workstations and our CICD.

This PR sums up the necessary changes in order for bazel-deps to be able to do that.

Just to be clear:

• the authFile support in dependencies.yaml is not going away, it's still the dominant setting
• in absence of authFile, _get_auth() will try to get the contents of ~/.netrc or if it doesn't exist return an empty dictionary

Also, this is my first time writing Scala code. Please please please please please tell me if I wrote the assembly of jarArtifactImpl with the variable part in between correctly.

[bartoszkosiorek]

@non Do you know who could take a look at this review?
It is resolving my long standing issue with authentication.