Use singleton instances, not just singleton type.

This is crucial to avoid creating a huge number of invalidations in
Base code. Using SnoopCompile (on Julia master ~v1.6):
```julia-repl
julia> using SnoopCompileCore

julia> invs = @Snoopr using HDF5;

julia> using SnoopCompile
```

**Before:**
```julia-repl
julia> trees = invalidation_trees(invs)
1-element Vector{SnoopCompile.MethodInvalidations}:
 inserting getproperty(::Type{H5E}, sym::Symbol) in HDF5 invalidated:
   backedges: 1: superseding getproperty(x::Type, f::Symbol) in Base at Base.jl:28 with MethodInstance for getproperty(::DataType, ::Symbol) (1220 children)
   3 mt_cache
```

**After:**
```julia-repl
julia> trees = invalidation_trees(invs)
SnoopCompile.MethodInvalidations[]
```

Author: jmert

src/deprecated.jl

@@ -226,7 +226,7 @@ end
 @deprecate exists(parent::Union{File,Group,Dataset,Datatype,Attributes}, path::String) Base.haskey(parent, path) false
 
 for pseudomod in (H5, H5D, H5E, H5F, H5FD, H5I, H5L, H5O, H5P, H5R, H5S, H5T, H5Z)
-    modsym = nameof(pseudomod)
+    modsym = nameof(typeof(pseudomod))
     for C in propertynames(pseudomod)
         constname = Symbol(modsym, :_, C)
         @eval @deprecate_binding($constname, $modsym.$C, false)

src/macros.jl

@@ -89,26 +89,26 @@ macro defconstants(prefix::Symbol, expr::Expr)
     # Push the imports into the definition body
     pushfirst!(defbody, Expr(:import, imports...))
 
-    prefix   = esc(prefix)
-    innermod = esc(innermod)
+    eprefix = esc(prefix)
+    einnermod = esc(innermod)
+    qprefix = QuoteNode(prefix)
     block = quote
-        Base.@__doc__(struct $prefix end)
-        baremodule $innermod
+        baremodule $einnermod
+            struct $eprefix end
             $(defbody...)
         end
-        function Base.propertynames(::Type{$prefix})
+        const $eprefix = $einnermod.$prefix()
+        function Base.propertynames(::$einnermod.$prefix)
             return $((symbols...,))
         end
-        function Base.getproperty(::Type{$prefix}, sym::Symbol)
+        function Base.getproperty(::$einnermod.$prefix, sym::Symbol)
             $(getbody...)
-            return getfield($prefix, sym)
         end
     end
     if !isempty(setbody)
         setfn = quote
-            function Base.setproperty!(::Type{$prefix}, sym::Symbol, value)
+            function Base.setproperty!(::$einnermod.$prefix, sym::Symbol, value)
                 $(setbody...)
-                return setfield!($prefix, sym, value)
             end
         end
         append!(block.args, Base.remove_linenums!(setfn).args)