File does not exist with `pkger`

[jufemaiz]

Hi team, love sql-migrate but have found an issue working with it and using the https://github.com/markbates/pkger/ solution for embedding static files.

Like #167 we are seeing issues related to file not found when traversing the files in the migration directory.

The steps used in sql-migrate are:

• FindMigrations is called on a HttpFileSystemMigrationSource https://github.com/rubenv/sql-migrate/blob/master/migrate.go#L232
• this then calls findMigrations (ref: https://github.com/rubenv/sql-migrate/blob/master/migrate.go#L247) and sets the directory to the base of the file system and then traverses its contents at that level (ref: https://github.com/rubenv/sql-migrate/blob/master/migrate.go#L260-L269)
• this iteration then calls out to migrationsFromFile (ref: https://github.com/rubenv/sql-migrate/blob/master/migrate.go#L277) which then also calls open on the filename of the file being iterated over. This is where the problem is surfacing. Because the filepath doesn't have the leading / in the string, it is failing to find the file.

Changing this to the following resolves this issue:

file, err := dir.Open(fmt.Sprintf("/%s"), info.Name())