avoid loading Pkg unless necessary

src/LazyArtifacts.jl

@@ -9,7 +9,60 @@ using Artifacts: Artifacts,
 export artifact_exists, artifact_path, artifact_meta, artifact_hash,
        select_downloadable_artifacts, find_artifacts_toml, @artifact_str
 
-# define a function for satisfying lazy Artifact downloads
-using Pkg.Artifacts: ensure_artifact_installed
+using Base.BinaryPlatforms: AbstractPlatform, HostPlatform
+
+"""
+    ensure_artifact_installed(name::String, artifacts_toml::String;
+                              platform::AbstractPlatform = HostPlatform(),
+                              pkg_uuid::Union{Base.UUID,Nothing}=nothing,
+                              verbose::Bool = false,
+                              quiet_download::Bool = false,
+                              io::IO=stderr)
+
+Ensures an artifact is installed, downloading it via the download information stored in
+`artifacts_toml` if necessary.  Throws an error if unable to install.
+"""
+function ensure_artifact_installed(name::String, artifacts_toml::String;
+                                   platform::AbstractPlatform = HostPlatform(),
+                                   pkg_uuid::Union{Base.UUID,Nothing}=nothing,
+                                   verbose::Bool = false,
+                                   quiet_download::Bool = false,
+                                   io::IO=stderr_f())
+    meta = artifact_meta(name, artifacts_toml; pkg_uuid=pkg_uuid, platform=platform)
+    if meta === nothing
+        error("Cannot locate artifact '$(name)' in '$(artifacts_toml)'")
+    end
+
+    return ensure_artifact_installed(name, meta, artifacts_toml;
+                                    platform, verbose, quiet_download, io)
+end
+
+function ensure_artifact_installed(name::String, meta::Dict, artifacts_toml::String;
+                                   platform::AbstractPlatform = HostPlatform(),
+                                   verbose::Bool = false,
+                                   quiet_download::Bool = false,
+                                   io::IO=stderr_f())
+
+    hash = SHA1(meta["git-tree-sha1"])
+    if !artifact_exists(hash)
+        # loading Pkg is a bit slow, so we only do it if we need to
+        # and do it in a subprocess to avoid precompilation complexity
+        return read(`$(Base.julia_cmd()) -e '
+            pkg = Base.require_stdlib(Base.PkgId(Base.UUID("44cfe95a-1eb2-52ea-b672-e2afdf69b78f"), "Pkg"));
+            Pkg.try_artifact_download_sources(
+                $(repr(name)),
+                $(repr(hash)),
+                $(repr(meta)),
+                $(repr(artifacts_toml));
+                $(repr(platform)),
+                $(repr(verbose)),
+                $(repr(quiet_download)),
+                $(repr(io))
+            )'
+        `, String)
+    else
+        return artifact_path(hash)
+    end
+end
 
 end