Problem: Gitlab has too frequent outages, which break automation.
Solution: Drop all Gitlab-hosted parsers (two of which have been
unmaintained for years).
Problem: Tracking parser revision in lockfile and allowing override
through the parsers module complicates the code. In addition, only
revision changes are handled robustly, not changes to other installation
info.
Solution: Track parser revision in the parsers module directly. Reload
parser table on every install or update call. Support modifying parser
table in a `User TSUpdate` autocommand.
