perf: cache depsCacheDirPrefix value for isOptimizedDepFile

[sapphi-red]

Description

isOptimizedDepFile was calling getDepsCacheDirPrefix every time.
So normalizePath and path.resolve was called every time.
Also this potentially causes gc as many string will be created.

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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[patak-dev]

Nice!