fix: Ensure sqlx reference is unambiguous

[hmac]

Prior to this change, I see errors such as

error[E0659]: `sqlx` is ambiguous
  --> database-schema-0.1.1/src/sqlite/mod.rs:28:5
   |
28 | use sqlx::fetch_structure_sql;
   |     ^^^^ ambiguous name
   |
   = note: ambiguous because of multiple potential import sources
   = note: `sqlx` could refer to a crate passed with `--extern`
   = help: use `::sqlx` to refer to this crate unambiguously
note: `sqlx` could also refer to the module defined here