swift-package-manager

[Basic] Start sketching FSProxy type.

 - This is intended as an abstraction around file system access. It can be used
   to proxy the access, to provide virtual file system behaviors to subsystems,
   and for easier testing of components.

 - The goal is not to provide a fully general virtual file system interface
   supporting all possible behaviors (in particular, the protocol requires
   synchronous operations currently), but is oriented towards the needs of the
   package manager.

 - This is a work in progress...