[DRE-maint] Bug#919346: ruby-data-migrate: Missing tasks/databases.rake file

[Andrew Lee (=?UTF-8?Q?=E6=9D=8E=E5=81=A5=E7=A7=8B?=)]

Package: ruby-data-migrate
Version: 3.2.2-1
Severity: important

Dear Ruby team,

Just noticed that the `tasks/databases.rake` are exist in the source
package, but doesn't get installed in the binary package.

So that I got following errors:
```
root at obs-api:/# /usr/share/obs/api/script/rake-tasks.sh setup
rake aborted!
LoadError: cannot load such file --
/usr/lib/ruby/vendor_ruby/data_migrate/../../tasks/databases.rake
/usr/share/obs/api/Rakefile:7:in `<top (required)>'
(See full trace by running task with --trace)
```

The problem is the tasks/ doesn't get installed. I did a hack on the
package to copy over tasks/ into lib/ at the build time to solved this.

However I am not sure if this is the proper way to handle the missing
installed file for ruby package. Please help to review my merge
request:
 https://salsa.debian.org/ruby-team/ruby-data-migrate/merge_requests/1

Best regards,
-Andrew