Use lowercase directory names

[AlekSi]

Description

For consistency.

Readiness checklist

• I added tests for new functionality or bugfixes.
• I ran task all, and it passed.
• I added/updated comments for both exported and unexported top-level declarations (functions, types, etc).
• I checked comments rendering with task godocs.
• I ensured that the title is good enough for the changelog.
• (for maintainers only) I set Reviewers (@FerretDB/core), Assignee, Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

[vercel]

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Name	Status	Preview	Updated
ferret-db	✅ Ready (Inspect)	Visit Preview	Nov 16, 2022 at 11:31AM (UTC)

[AlekSi]

Sorry, it is broken, one moment

[codecov]

Codecov Report

Merging #1408 (4445f0d) into main (a30ba77) will decrease coverage by 0.09%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1408      +/-   ##
==========================================
- Coverage   67.40%   67.31%   -0.10%     
==========================================
  Files         279      279              
  Lines       13353    13353              
==========================================
- Hits         9001     8988      -13     
- Misses       3426     3441      +15     
+ Partials      926      924       -2     

Impacted Files	Coverage Δ
internal/handlers/tigris/tigrisdb/errors.go	32.14% <0.00%> (-10.72%)	⬇️
internal/handlers/tigris/msg_create.go	67.60% <0.00%> (-7.05%)	⬇️
internal/handlers/tigris/tigrisdb/collections.go	54.28% <0.00%> (-5.72%)	⬇️
internal/util/version/version.go	48.52% <0.00%> (-4.42%)	⬇️
internal/handlers/common/filter.go	84.65% <0.00%> (ø)

Flag	Coverage Δ
integration	63.41% <ø> (-0.10%)	⬇️
mongodb	14.03% <ø> (-0.03%)	⬇️
pg	52.63% <ø> (-0.03%)	⬇️
tigris	35.22% <ø> (-0.10%)	⬇️
unit	26.07% <ø> (-0.03%)	⬇️

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