Register builtin builders

[edolstra]

Motivation

This makes the builtin builders (builtin:fetchurl etc) register themselves at runtime, so that DerivationBuilderImpl::runChild() doesn't need to know about them.

Context

I want to add a builtin:fetch-tree builtin builder (DeterminateSystems#49). However, this would require libstore to depend on libfetchers, which would be a cyclic dependency. So we need to be able to define builtin builders outside of libstore.

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[xokdvium]

Should registering multiple instances with the same name be ok and overwrite the value? Wouldn't that allow plugins to override standard builders?

[roberth]

I want to add a builtin:fetch-tree builtin builder (DeterminateSystems#49).

We should do #9259 as a prerequisite so that

• derivations are equivalent regardless of how their content-addressed inputs are produced
• buildTime = true is not needed

[Mic92]

I want to add a builtin:fetch-tree builtin builder (DeterminateSystems#49).

We should do #9259 as a prerequisite so that

* derivations are equivalent regardless of how their content-addressed inputs are produced

* `buildTime = true` is not needed

Is this blocking this PR in any way? I like that src/libstore/unix/build/derivation-builder.cc no has less knowledge over builtin builders. Also make potential plugins easier to build.

[edolstra]

@Mic92 No, that would be discussion for a future PR. This PR is just refactoring to provide the necessary infrastructure for registering builtin builders.