[refactor] unify Database & ExternalIDs

These types are the same, except for Database.Empty

I managed to have backwards compatibility with the frontend format,
hence the frontend doesn't need any changes.
4 jobs for 224-dev-understanding-ngrams in 20 minutes and 51 seconds (queued for 6 seconds)
Status Job ID Name Coverage
  Cabal
failed #16951
cabal

00:20:51

 
  Stack
skipped #16952
stack
 
  Bench
skipped #16953
allowed to fail manual
bench
 
  Test
skipped #16954
test
 
Name Stage Failure
failed
cabal Cabal
ERROR! hash mismatch between expected cabal.project and the one computed by cabal2stack.
Please update the hashes inside the './bin/update-project-dependencies' file.
+ actual_cabal_project_freeze_hash=32310c4d4e7b4679dcb90dcfcd0d6d1b175dbf885a77ffddca16d422998a521c
+ [[ ac293a4c66092996bc85fbf14ef34b7cce3ed5b0612ceb9e1a5f395059631e0b != 1abcdd99d5d50660e640be8a340c90331a84ef266d174c7ca6099c1c04ef65ea ]]
+ echo -e '\e[31mERROR! hash mismatch between expected cabal.project and the one computed by cabal2stack.\e[0m'
+ echo -e '\e[33mPlease update the hashes inside the '\''./bin/update-project-dependencies'\'' file.\e[0m'
+ exit 1
Cleaning up project directory and file based variables
ERROR: Job failed: exit code 1