Arcana

Fixes importing of extension CLI modules

* Adds more structure to the test dataset blueprints

* Removes dependency on arcana-dirtree package for tests
* Also moves FileGroup out to fileformats-core package

This release works towards resolving circular imports between dependencies in the core unittests

This is the first release of the core Arcana package separate from the other (now "extension") packages.
I have taken this opportunity to reset the versioning back from v2.0b to prerelease v0.8 (v0.7 referring to the rewritten monolithic package). Arcana v2 will just be a "metapackage" that installs the standard components

* arcana-core
* arcana-dirtree
* arcana-xnat
* arcana-bids
* arcana-spaces