.arcconfig

path: introduce a `path://` syntax to reference other path name

This make it easier for a path to reuse the same location of another path with different parameter. This is useful to create path "alias" with common config option.

This will become very useful to create path that reference a list of other path.

This changeset focus on implemented the basic feature, future changesets will
deal with various error management (and associated testing).

Differential Revision: https://phab.mercurial-scm.org/D10263

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