Don't download artifact when overwritten via Overrides.toml

[carstenbauer]

Came up over at MKL.jl here.

Currently, if you use Overrides.toml to point Julia to a custom artifact Pkg will still eagerly download the original artifact and then just not use it afterwards.

Example: IntelOpenMP_jll:

Overrides.toml

[1d5cc7b8-4909-519e-a0f8-d0f5ad9712d0]
IntelOpenMP = "/path/to/my/custom/artifact/" # contains lib/libiomp5.so and  lib/libiompstubs5.so

Pkg still downloads the original artifact:

(@v1.8) pkg> add IntelOpenMP_jll
   Resolving package versions...
  Downloaded artifact: IntelOpenMP
    Updating `~/.julia/environments/v1.8/Project.toml`
  [1d5cc7b8] + IntelOpenMP_jll v2018.0.3+2
    Updating `~/.julia/environments/v1.8/Manifest.toml`
  [1d5cc7b8] + IntelOpenMP_jll v2018.0.3+2
Precompiling project...
  1 dependency successfully precompiled in 1 seconds (9 already precompiled)

➜  bauerc@ln-0001 .julia/artifacts  ls
947793e42b663bacd09f00d96aa96a47095f3b1c  Overrides.toml
➜  bauerc@ln-0001 .julia/artifacts  cd 947793e42b663bacd09f00d96aa96a47095f3b1c/lib/; ls
libiomp5.so  libiompstubs5.so

But doesn't use it (that's desired of course 😄):

julia> IntelOpenMP_jll.libiomp_path
"/path/to/my/custom/artifact/lib/libiomp5.so"

Doesn't happen for lazy artifacts

If an artifact is marked as lazy = true the unnecessary download of the original artifact doesn't happen, as desired. For example, MKL_jll.jl is lazy.

Desired behaviour:
IMO, if an artifact is overwritten in an Overrides.toml, the original artifact should not be downloaded. Is there something we can do about this?

[mkitti]

I have encountered interest in this feature downstream. This would make the option to use conda-forge dependencies easier, for example, which may be necessary in cases where cross-compilation is not available or when interop is paramount.

I thought Artifacts was moving from to Julia proper and out of Pkg.jl. Is this is the right place to have this issue?