Searching for exact user ids in the user directory fails to match

This issue has been migrated from [#3631](https://github.com/matrix-org/synapse/issues/3631).

---

### Description

``curl 'https://matrix.half-shot.uk/_matrix/client/r0/user_directory/search?' --data '{"search_term":"@Half-Shot:half-shot.uk"}'``

Results in: ``{
"limited": false,
"results": []
}``

Whereas:

``curl 'https://matrix.half-shot.uk/_matrix/client/r0/user_directory/search?' --data '{"search_term":"@Half-Shot:half-shot"}'``

Comes back with many results, including the one I was after.

### Version information

- **Version**: Synapse/0.33.0