Move lockfile to its own module and general refactor

[mark-thm]

Issue

Closes #27.

rules_multitool suggests using a $schema field but this CLI doesn't support that field.

Summary

Refactor lockfile handling to its own module, update struct names for legibility, and add support for specifying $schema. Additionally, validate $schema matches expectation, always serialize a value for $schema, and add some debug prints for tool updates. Last, move regex initialization into a static block.