Fix #582

This PR reinstates the existing behaviour where users can register Go migrations manually without having the corresponding files on disk, or in the embedded filesystem. This is accomplished using the goose.AddNamedMigration.. set of functions.

// If Go migrations have been registered globally, with [goose.AddNamedMigration...], but there are
// no corresponding .go files in the filesystem, add them to the migrations slice.

Fix #553

This PR also allows registering Go migrations, while filtering them down to only those found in the filesystem based on the specific directory.

// If Go migrations have been registered, and there are .go files in the filesystem dirpath, ONLY
// include those in the migrations slices.

We continue to preserve the behaviour where a valid looking .go file doesn't have a corresponding registered migration raises an error.

// Lastly, if there are .go files in the filesystem but they have not been registered, raise an
// error. This is to prevent users from accidentally adding valid looking Go files to the migrations
// folder without registering them.

I've also added a number of tests to capture all this behaviour. See TestCollectMigrations for more details.