A more convenient interface to get a throwaway empty credentials source
for use in tests, which doesn't interact at all with the real CLI
configuration directory.
This was a leftover from the migration of these types from the main
package, but we don't actually need or want this here because this
particular detail is still handled by the main package, and because the
cliconfig package must not depend on the command package in order to avoid
an import cycle.
This new implementation is not yet used, but should eventually replace the
technique of composing together various types from the svchost/auth
package, since our requirements are now complex enough that they're more
straightforward to express in direct code within a single type than as
a composition of the building blocks in the svchost/auth package.
This is just a wholesale move of the CLI configuration types and functions
from the main package into its own package, leaving behind some type
aliases and wrappers for now to keep existing callers working.
This commit alone doesn't really achieve anything, but in future commits
we'll expand the functionality in this package.
