Cannot pass a netrc from current repo

### Description of the problem / feature request:
Currently, it is not possible to use a file from the current repository as netrc file for http rules, unless the full path to the repo is known in advance.

### Proposed solution
Use `label` as type for the `netrc` attribute in `http_file` and `http_archive` rules. Current full paths will have to be exposed through `new_local_repository` workspace targets. This might require an incompatible change flag.

### What's the output of `bazel info release`?

release 2.0.0

Start by tracing the netrc attributes on the http_file and http_archive rules, then examine how current full paths are exposed through new_local_repository workspace targets. Check the compatibility implications of changing netrc to a label, including whether an incompatible change flag is required. Done means repository-local netrc files can be passed without knowing the repository's full path.

Written by the indexing model from the issue text.