refactor: rename some mod.rs to <MOD_NAME>.rs

[discord9]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

rename mod.rs to each <module_name>.rs according to https://doc.rust-lang.org/reference/items/modules.html#module-source-filenames 's suggestion(Except for files in tests/ folder which are still using mod.rs)

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.

[codecov]

Codecov Report

Merging #784 (072bbfc) into develop (e07791c) will increase coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           develop     #784    +/-   ##
=========================================
  Coverage    85.42%   85.43%            
=========================================
  Files          414      415     +1     
  Lines        54230    54432   +202     
=========================================
+ Hits         46328    46506   +178     
- Misses        7902     7926    +24     

Flag	Coverage Δ
rust	85.43% <ø> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/script/src/python/builtins.rs	87.12% <ø> (ø)
src/servers/src/mysql/handler.rs	59.84% <0.00%> (-4.55%)	⬇️
src/frontend/src/frontend.rs	38.88% <0.00%> (-1.12%)	⬇️
src/common/query/src/logical_plan/accumulator.rs	92.64% <0.00%> (-0.51%)	⬇️
src/query/src/query_engine/state.rs	85.05% <0.00%> (-0.38%)	⬇️
src/cmd/src/standalone.rs	49.54% <0.00%> (-0.23%)	⬇️
src/servers/src/lib.rs	100.00% <0.00%> (ø)
src/query/src/datafusion/planner.rs	66.66% <0.00%> (ø)
src/servers/src/interceptor.rs	81.08% <0.00%> (ø)
src/sql/src/parser.rs	81.40% <0.00%> (+0.04%)	⬆️
... and 6 more

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

[waynexia]

👍 LGTM

[evenyag]

Thanks! This also fixes a task in #555