Replace directives pointing to local paths prevent `go install` from
working. Use published module versions (sqlmigrate v1.0.2, shmigrate
v1.0.2) so users can install with:
go install github.com/therootcompany/golib/cmd/sql-migrate/v2@latest
Migration{ID, Name} is the identity type returned by Up/Down/Latest/Drop.
Script{Migration, Up, Down} holds collected SQL content from Collect().
Migrator interface now takes SQL as a separate parameter:
ExecUp(ctx, Migration, sql string)
ExecDown(ctx, Migration, sql string)
This separates identity from content — callers that track what ran
don't need to carry around SQL strings they'll never use.
Updates shmigrate to match (ignores the sql parameter, references
files on disk instead).
