Move platform-specific code out of DerivationBuilder

[edolstra]

Motivation

This PR moves almost all of the Linux/Darwin-specific code out of DerivationBuilderImpl and into separate files/subclasses. I.e. we have the following class hierarchy:

DerivationBuilderImpl
|- LinuxDerivationBuilder
+- DarwinDerivationBuilder

DerivationBuilderImpl itself implements generic unsandboxed Unix building, but has virtual methods that allow the subclasses to provide sandboxing.

LinuxDerivationBuilder performs namespaced/chroot builds on Linux. It is not used on Linux if you have sandboxing disabled. (Currently the seccomp/personality code is part of this class, so we may want to split the class hierarchy a bit further.)

makeDerivationBuilder() instantiates the right class based on the OS type and whether sandboxing is enabled.

In the future, we should split the generic Unix code (e.g. user allocation, runChild(), pseudo-terminal allocation) from the platform-independent code (e.g. registerOutputs()), so we could have something like this:

DerivationBuilder
|- UnixDerivationBuilder
|   |- LinuxDerivationBuilder
|   +- DarwinDerivationBuilder
+- WindowsDerivationBuilder

Context

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[Ericson2314]

Fantastic! Thanks for much for doing this @edolstra!

[Ericson2314]

I'll let this slide for now :)

[cole-h]

Out of curiosity, what would "do this properly" look like? A separate file that has these includes and the function definition, or...?

[edolstra]

Yeah, we'd have to move DerivationBuilderImpl into a header. But putting an impl class in a header kind of defeats the purpose. And there are probably some more refactorings that we want to do (like splitting DerivationBuilder and UnixDerivationBuilder) but we can do that in a future PR.