Update sqlite3 package, general maintenance

[simolus3]

This updates dependencies to the latest version. In particular, it updates to version 3.x of the sqlite3 package which now relies on build hooks instead of manual DynamicLibrary.open calls. Thus, we can remove all open.overrideFor calls from our code (good riddance!).

Additionally, this:

• Migrates from melos to a pub workspace, giving us a faster pub get in CI and making it impossible to have mismatched versions (drift_sqlite_async can't have sqlite_async: ^0.13.0 in its dependencies when the current version from the workspace is 0.14.0 for example).
• Adds automated publishing to pub.dev on matching version tags.
• Refactors the test CI to cache pub packages across runs.
• Updates readmes to remove outdated information like the defunct pub popularity score.