In Sbt (prior to 2.0) a sbt key not scoped to `ThisBuild` will always be at the project level, rather than at the build level.
In order to do some things, like use a private repository configured by sbt with sbt-scalafix, we need the credentials scope to the build, otherwise the credentials show as non-existent.
Instead of having to manually stub packages that should not be
downloaded we instead automatically stub all packages (except a small
list of whitelisted ones). Tests can re-introduce the real package by
using the `realPkgs` module argument.
This will cache the output of `passwordCommand` per authentication
realm.
Context: the `credentials` key in `sbt` is a `TaskKey[Seq[Credentials]]`.
In `sbt`, tasks are evaluated on-demand and their output is not cached.
This particular key is referenced by all submodules in a project. When
the command is relatively expensive (e.g.: `pass show foo`), this
results in several seconds of delay when doing basic things like
`compile` or `test` which makes this unusable without some kind of
caching.
