removeReferencesToVendoredSources: cache vendorDir crawling

[ipetkov]

Motivation

There is no reason to repeatedly crawl the vendor directories for each installed file, we can easily cache the results once up front.

Fixes #620

Checklist

• added tests to verify new behavior
• added an example template or updated an existing one
• updated docs/API.md (or general documentation) with changes
• updated CHANGELOG.md

[9999years]

I think xargs --no-run-if-empty signIfRequired is introducing an error to my build because signIfRequired is a shell function and not an executable, so xargs can't find it:

https://github.com/NixOS/nixpkgs/blob/7187b3ee8e2687106e8982b02d2890689471579e/pkgs/os-specific/darwin/signing-utils/utils.sh#L38-L43

xargs: signIfRequired: No such file or directory

Maybe we can work around by doing something like this:

https://github.com/NixOS/nixpkgs/blob/7187b3ee8e2687106e8982b02d2890689471579e/pkgs/os-specific/darwin/signing-utils/auto-sign-hook.sh#L19-L21