*: replace os.FileInfo with os.DirEntry where possible

[shubhamdhama]

Previously, in a few places, we converted os.DirEntry to os.FileInfo by
calling entry.Info(), which triggers a stat syscall for each entry.

Most callers only needed the entry name, which is directly available from
os.DirEntry via Name(). This patch switches to os.DirEntry throughout the
codebase, avoiding unnecessary stat syscalls. In the few places where file
size or mode is needed, we now call Info() only when required.

Epic: none
Release note: None
Fixes: none

[cockroach-teamcity]

This change is 

[stevendanna]

I wonder: Would it be worth following up with a few of these callsites to see if they need to use this function at all?

I checked 2 of the call site and it appeared they only called Name() and IsDir() on the resulting FileInfo. Name and IsDir is available from the DirEntry's returned by the os.ReadDir call, so we could save some extra stat calls.

[shubhamdhama]

@stevendanna, right, that's a good catch. I think I can do that clean-up as a follow-up for the security_assets.go and profilestore_test.go

[stevendanna]

👍 This is a nice cleanup even without the follow-up.

[shubhamdhama]

@stevendanna I pushed another commit following this suggestion: #157257 (review). This change makes fileutil.ReadDir redundant, so I've removed it. I'm planning to squash the first commit into the second (keeping only the second one) and update the PR title and description accordingly. Would appreciate another look when you get a chance.

[stevendanna]

Thanks for following up!

[shubhamdhama]

TFTR!

bors r=stevendanna

[craig]

Build succeeded:

• unit_tests
• local_roachtest_fips
• local_roachtest
• linux_amd64_build
• acceptance
• lint
• examples_orms
• check_generated_code
• linux_amd64_fips_build
• docker_image_amd64