Set up better dependency checks

[dbussink]

In order to avoid future regressions around some key areas used as libraries, let's make sure we have some depguard checks in place that block some regularly occuring cases.

This is not green until we merge the regression from v22 in #18507

It also fixes one violation which is not as problematic as #18507 and has existed for longer already, so it's ok to only fix that on main moving forward.

Related Issue(s)

Part of fixing #18506 and important for #14717

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.49%. Comparing base (21df269) to head (46bc156).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
go/cmd/vtgate/cli/plugin_auth_clientcert.go	0.00%	3 Missing ⚠️
go/vt/vtgate/plugin_mysql_server.go	0.00%	2 Missing ⚠️
go/mysql/auth_server_clientcert.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #18508   +/-   ##
=======================================
  Coverage   67.49%   67.49%           
=======================================
  Files        1607     1607           
  Lines      263045   263046    +1     
=======================================
+ Hits       177540   177545    +5     
+ Misses      85505    85501    -4     

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