Regression: fix compilation performance on Windows

[OlegYch]

Caches isDirectory calls

Too many of them were added in 607e4d5 and this degraded compilation performance by up to 100% on Windows

Fixes #19924
backport to release-3.4.2

[OlegYch]

signed CLA

[som-snytt]

We need a "platform linter".

[jchyb]

Removing create() and delete() directories is dangerous - dotty.tools.io is a stable library that is being used in other projects, and those methods were implemented for PlainDirectory and PlainFile. With that said I looked through GitHub for all of the usages of those two, and they do not appear to be used anywhere (e.g. PlainDirectory is used in twirl and scalafix, but only to pass output directory to the compiler, PlainFile is only used in the compiler). With that in mind, and considering the performance benefit, I believe this should be merged

Nicolas is unavailable, however all of the problems raised during the review were resolved by the PR creator

[jchyb]

Looks like in between the reviews I embarrassingly added a PlainFile.delete() method (which I of course forgot about), which of course I missed during my check right now, meaning we have a failing main. I will revert the revert after removing that delete method for the next release. Apologies for the inconvenience!

[OlegYch]

is it possible to add this to 3.4.3 ?

[SethTisue]

is it possible to add this to 3.4.3 ?

(I don't make these decisions myself but) that seems implausible, as 3.4.3 wasn't even a planned release, it's a special hotfix release for a particular problem affecting compatibility, as per https://contributors.scala-lang.org/t/3-4-3-release-thread/6718

for some time now project attention has been on the 3.3.x and 3.5.x, and forthcoming 3.6.x series

also 3.5.0 is imminent, I hope you will be able to upgrade to it

[OlegYch]

@SethTisue i don't think this ended up in 3.5.0
but it seems to be present in 3.5.1-RC2
just a heads up