generate_completion_cache: use sqlite instead of text files

This will really increase performace for bash-completion.
```
$ time sqlite3 packages.db "select pkg from available WHERE pkg LIKE \"tex%\";"
real    0m0.018s
user    0m0.008s
sys    0m0.010s
```
```
$ time grep -E ^tex /var/cache/dnf/available.cache
real    0m0.112s
user    0m0.017s
sys    0m0.043s
```
Signed-off-by: Igor Gnatenko <i.gnatenko.brain@gmail.com>