Originally the hope was to use the afero filesystem abstraction for all
loader operations, but since we install modules using go-getter we cannot
(without a lot of refactoring) support vfs for installation.
The vfs use-case is for reading configuration from plan zip files anyway,
and so we have no real reason to support installation into a vfs. For now
at least we will just add the possibility that a loader might not be
install-capable. At the moment we have no non-install-capable loaders, but
we'll add one later once we get to loading configuration from plan files.
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