These functions provide a nice interface for determining the
currently-running package's UUID and VersionNumber, identified by loaded
`Module`.  We explicitly do not include a lookup by name, in expectation
of potential future naming clashes due to having multiple packages with
the same name but different UUIDs.

This implements functionality and tests for a new `Scratch`
subsystem in `Pkg`; analogous to the `Artifacts` added in 1.3, this
provides an abstraction for a mutable datastore that can be explicitly
lifecycled to an owning package, or shared among multiple packages.

Closes #796

Preferences provides a simple package configuration store; packages can
store arbitrary configurations into `Dict` objects that get serialized
into their active `Project.toml`.  Depot-wide preferences can also be
stored within the `prefs` folder of a Julia depot, allowing for default
values to be passed down to new environments from the system admin.