Various other packages currently have somewhat-complicated helpers for
loading configuration that involve building temporary directories on disk
and filling them with source files, but "package configs" itself can
provide similar helpers without so much complexity by relying on its own
internals a little to bypass details that are not so important for simple
test cases.
This introduces some new helpers but doesn't yet introduce any callers of
them because these helpers in particular are aimed at an experimental new
approach to config evaluation that makes it some other package's problem
to assemble the configuration tree, and so it just wants individual
configs.Module objects instead of configs.Config trees.
Signed-off-by: Martin Atkins <mart@degeneration.co.uk>
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