Single purl lookup on `packages` endpoint can take up to 20 seconds

For example a call on `https://public.vulnerablecode.io/api/packages/` with the following payload: `{'purl': 'pkg:alpine/libxml2@2.9.8-r1'}`

```
from scanpipe.pipes import vulnerablecode
from timeit import default_timer as timer

start_time = timer()
vulnerabilities = vulnerablecode.get_vulnerabilities_by_purl("pkg:alpine/libxml2@2.9.8-r4")
print(timer() - start_time)
# -> 19.21
```