feat(JLLEnvs): try to load local Artifacts.toml if existed

This commit provides two changes:

- By default, use the stable release version of `Artifacts.toml` provided by
  binarybuilderbase. previously it uses the master version, which can sometimes
  be breaking and troublesome.
- it utilizes the local disk cache and thus pkg server so that we don't need to
  fetch resources from GitHub.com and thus is more friendly to users behind the
  firewall.

As a consequence of getting rid of the need to download from remote server,
this commit makes `@time using Clang` much faster: from 1.7s to 0.4s in Julia
1.7, macOS Intel i9-9880H.

Project.toml

@@ -8,6 +8,7 @@ Clang_jll = "0ee61d77-7f21-5576-8119-9fcc46b10100"
 Downloads = "f43a241f-c20a-4ad4-852c-f6b1247861c6"
 Pkg = "44cfe95a-1eb2-52ea-b672-e2afdf69b78f"
 TOML = "fa267f1f-6049-4f14-aa54-33bafae1ed76"
+UUIDs = "cf7118a7-6976-5b1a-9a39-7adc72f591a4"
 
 [compat]
 CEnum = "0.4"

src/shards/JLLEnvs.jl

@@ -3,18 +3,41 @@ module JLLEnvs
 using Pkg
 using Pkg.Artifacts
 using Downloads
+using UUIDs
 
 include("utils.jl")
 
 const JLL_ENV_SHARDS_URL = "https://raw.githubusercontent.com/JuliaPackaging/BinaryBuilderBase.jl/master/Artifacts.toml"
 const JLL_ENV_SHARDS = Dict{String,Any}()
 
+function _checked_import(pkgid)
+    Base.root_module_exists(pkgid) && return Base.root_module(pkgid)
+    Base.require(pkgid)
+end
+
 function __init__()
     if haskey(ENV, "JULIA_CLANG_SHARDS_URL") && !isempty(get(ENV, "JULIA_CLANG_SHARDS_URL", ""))
         merge!(JLL_ENV_SHARDS, Artifacts.load_artifacts_toml(ENV["JULIA_CLANG_SHARDS_URL"]))
-    else
-        merge!(JLL_ENV_SHARDS, Artifacts.load_artifacts_toml(Downloads.download(JLL_ENV_SHARDS_URL)))
+        return
     end
+
+    # If possible, try to load stable version of Artifacts.toml from local disk cache and thus
+    # makes `using Clang` faster. This requires `BinaryBuilderBase` available in `DEPOT_PATH`,
+    # e.g., via `(@v1.7) pkg> add BinaryBuilderBase`.
+    id_BinaryBuilderBase = Base.PkgId(UUID("7f725544-6523-48cd-82d1-3fa08ff4056e"), "BinaryBuilderBase")
+    artifact_file = ""
+    try
+        artifact_file = joinpath(pkgdir(_checked_import(id_BinaryBuilderBase)), "Artifacts.toml")
+    catch e
+        @debug e
+    end
+    if isfile(artifact_file)
+        merge!(JLL_ENV_SHARDS, Artifacts.load_artifacts_toml(artifact_file))
+    end
+    isempty(JLL_ENV_SHARDS) || return
+
+    @debug "Failed to load package `BinaryBuilderBase`, fetching from remote resources..."
+    merge!(JLL_ENV_SHARDS, Artifacts.load_artifacts_toml(Downloads.download(JLL_ENV_SHARDS_URL)))
 end
 
 const JLL_ENV_HOST_TRIPLE = "x86_64-linux-musl"