Add Artifacts as a standard library

Imports `Pkg.Artifacts` into its own stdlib, Artifacts.  Includes the loading
and locating pieces of `Pkg.Artifacts`, but not the pieces for downloading and
installing artifacts, which still reside within Pkg.  Scripts that want to
generate artifacts should still use `Pkg.Artifacts`, but scripts that just want
to use artifacts that have already been generated are free to simply
`using Artifacts`, which should cut down on time to first load by a nice chunk.
If a package attempts to `@artifact"foo"` on an artifact that does not exist
the `Artifacts` module will dynamically load `Pkg` and invoke the downloading
routines through an inference barrier.

Also adds two new (minor) features:

* Slash-indexing: by popular demand, supports indexing into files within
  artifacts in one convenient syntax: `@artifact"FFMPEG/bin/ffmpeg"`
  will automatically perform the `joinpath()` for you after looking up
  the `FFMPEG` artifact.

* Compile-time constancy; when `@artifact_str` is given a constant
  `String`, it optimizes the generated code by looking up the
  appropriate hash at compile-time and embedding just that hash into the
  resultant code, saving on both lookup time and objects that must be
  serialized to the eventual precompilation `.ji` file.

NEWS.md

@@ -102,6 +102,9 @@ Standard library changes
 * The `Pkg.BinaryPlatforms` module has been moved into `Base` as `Base.BinaryPlatforms` and heavily reworked.
   Applications that want to be compatible with the old API should continue to import `Pkg.BinaryPlatforms`,
   however new users should use `Base.BinaryPlatforms` directly. ([#37320])
+* The `Pkg.Artifacts` module has been imported as a separate standard library.  It is still available as
+  `Pkg.Artifacts`, however starting from Julia v1.6+, packages may import simply `Artifacts` without importing
+  all of `Pkg` alongside. ([#37320])
 
 #### LinearAlgebra
 * New method `LinearAlgebra.issuccess(::CholeskyPivoted)` for checking whether pivoted Cholesky factorization was successful ([#36002]).

base/sysimg.jl

@@ -47,6 +47,7 @@ let
             :Distributed,
             :SharedArrays,
             :TOML,
+            :Artifacts,
             :Pkg,
             :Test,
             :REPL,

contrib/generate_precompile.jl

@@ -65,6 +65,21 @@ if Distributed !== nothing
     """
 end
 
+Artifacts = get(Base.loaded_modules,
+          Base.PkgId(Base.UUID("56f22d72-fd6d-98f1-02f0-08ddc0907c33"), "Artifacts"),
+          nothing)
+if Artifacts !== nothing
+    precompile_script *= """
+    using Artifacts, Base.BinaryPlatforms
+    artifacts_toml = abspath($(repr(joinpath(Sys.STDLIB, "Artifacts", "test", "Artifacts.toml"))))
+    cd(() -> @artifact_str("c_simple"), dirname(artifacts_toml))
+    artifacts = Artifacts.load_artifacts_toml(artifacts_toml)
+    platforms = [Artifacts.unpack_platform(e, "c_simple", artifacts_toml) for e in artifacts["c_simple"]]
+    best_platform = select_platform(Dict(p => triplet(p) for p in platforms))
+    """
+end
+
+
 Pkg = get(Base.loaded_modules,
           Base.PkgId(Base.UUID("44cfe95a-1eb2-52ea-b672-e2afdf69b78f"), "Pkg"),
           nothing)

stdlib/Artifacts/Project.toml

@@ -0,0 +1,8 @@
+name = "Artifacts"
+uuid = "56f22d72-fd6d-98f1-02f0-08ddc0907c33"
+
+[extras]
+Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
+
+[targets]
+test = ["Test"]