Fix resolving inherit_gem in remote configs

[CvX]

When a file inherits from a url inheriting from a gem, like so:

# .rubocop.yml
inherit_from: http://example.com/default.yml

# http://example.com/default.yml
inherit_gem:
    somegem: default.yml

# somegem's default.yml
Layout/LineLength:
    Max: 48

Rubocop would then incorrectly try to resolve a URL like this:

http://example.com//private/var/folders/69/bjrqv8v1225f7yqnxknm3fb80000gn/T/d20200518-94245-6dqeew/work/gems/somegem/default.yml

With this change, local gem paths are no longer taken by mistake for relative remote paths.

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[bbatsov]

Thanks!