Cache JDKs in a central place

[Vampire]

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It would be nice if by default downloaded JDKs are cached in a central plac,e so that you don't have 10 times the same downloaded and unpacked JDK on your disk just because you build 10 projects. The Gradle user home would maybe be a good (default) choice for such a central place.

I know that I can currently configure the downloadDir, but for a central cross-project storage I think a bit more effort is needed than just to set that central place. Maybe something with locks and checksums or similar.

[jsixface]

I am using the Kotlin build script, and I can't pass in the downloadDir parameter since it's expecting a Groovy closure. The comment above is my primary reason to change the download dir, but I can't do that right now.

[Vampire]

closureOf { ... } or delegateClosureOf { ... } should probably work.