rules_envfile only supports files that are in a bazel package

[nateinaction]

Currently you can only include envfiles that are in a bazel package. It would be great if envfiles that are not in a bazel package were also supported.

envfile(
    name = "env",
    files = [
        "non-package/non-package.env",
        "//test-package:package.env"
    ]
)

bazel build //... currently fails with:

INFO: Repository env instantiated at:
  /Users/nate/code/github.com/nateinaction/rules_envfile/WORKSPACE:5:8: in <toplevel>
Repository rule envfile defined at:
  /Users/nate/code/github.com/nateinaction/rules_envfile/env.bzl:24:26: in <toplevel>
ERROR: An error occurred during the fetch of repository 'env':
   Traceback (most recent call last):
        File "/Users/nate/code/github.com/nateinaction/rules_envfile/env.bzl", line 16, column 35, in _envfile_impl
                path = repository_ctx.path(file)
Error in path: Not a regular file: /Users/nate/code/github.com/nateinaction/rules_envfile/external/non-package/non-package.env
ERROR: Error fetching repository: Traceback (most recent call last):
        File "/Users/nate/code/github.com/nateinaction/rules_envfile/env.bzl", line 16, column 35, in _envfile_impl
                path = repository_ctx.path(file)
Error in path: Not a regular file: /Users/nate/code/github.com/nateinaction/rules_envfile/external/non-package/non-package.env
ERROR: no such package '@env//': Not a regular file: /Users/nate/code/github.com/nateinaction/rules_envfile/external/non-package/non-package.env
INFO: Elapsed time: 0.134s
INFO: 0 processes.
FAILED: Build did NOT complete successfully (2 packages loaded)
    currently loading: test