sbt allows overriding the default repositories to use to resolve
dependencies. This is often used with proxies and/or private
repositories to host internal packages.
This change adds a `repositories` attribute to `sbt` to allow
specifying the values that will go in `~/.sbt/repositories` file.
To support the above change we also deprecate the `baseConfigPath`
option in favour of `baseUserConfigPath` which points one level higher
by default. This allows not using relative paths to refer to the
top-level configuration directory.
Also adds tests for the new option and the deprecation of the previous
one.
