Reorganize paperspec commands and pin dependency versions (#2)

Reviewed-on: #2

spec/00-introduction.md

@@ -82,8 +82,9 @@ The other folders help people use the specification.
 | `registry/` | Formats, rule IDs, stamp rules and the default stage requirements. | Yes |
 | `vectors/` | Test vectors, one case per line. | Yes |
 | `fixtures/` | Valid packages and records, invalid packages, and the keys that signed them. | Yes |
-| `tools/` | The programs that compute and check the vectors and fixtures, and load packages into SQLite. | No |
+| `src/` | The Python package `paperspec`: the reference implementation, and the `paperspec` command that computes and checks the vectors and fixtures and loads packages into SQLite. | No |
+| `tools/` | How to run those commands. | No |
 | `docs/` | Guides for people getting to know the formats. | No |
 
-The programs in `tools/` compute every expected value, so that none is typed by hand.
-They are not a library, and implementations conform to the chapters and the vectors.
+The reference implementation in `src/paperspec/` computes every expected value, so that none is typed by hand.
+Other repositories of the project import it, but implementations conform to the chapters and the vectors, not to it.