Reorganize paperspec commands and pin dependency versions (#2)

Reviewed-on: infra/paperspec#2

fixtures/src/README.md

@@ -1,6 +1,6 @@
 # Fixture sources
 
-Each source describes a document as a converter first read it, mistakes included, in a shorthand that `tools/build.py` expands.
+Each source describes a document as a converter first read it, mistakes included, in a shorthand that `paperspec build` expands.
 The build mints IDs, applies the listed corrections, lays out synthetic geometry, signs the listed receipts and writes the package.
 The shorthand is tooling, not a format.