Prevent extensions from blocking parallel pre-compilation

Previously our precompilation code was causing any dependencies of a
package A to wait on all of A's weakdeps to finish pre-compiling,
even if it can't actually load those weakdeps (or the extension itself)

This would lead to a pre-compile ordering like:
```
A        B
 \      / \
  Ext AB   \
     |     /
     C    /
      \  /
       D
```

Here, extension `C` cannot pre-compile in parallel with `Ext {A,B}` and
`B`, because it has to wait for `Ext {A,B}` to finish pre-compiling.
That happens even though `C` has no way to load either of these.

This change updates the pre-compile ordering to be more parallel,
reflecting the true place where `Ext {A,B}` can be loaded:
```
  A       B
 / \     / \
C   Ext AB  |
 \    |    /
  \-- D --/
```

which allows `C` to compile in parallel with `B` and `Ext{A,B}`

base/precompilation.jl

@@ -439,14 +439,48 @@ function precompilepkgs(pkgs::Vector{String}=String[];
     append!(project_deps, keys(filter(d->last(d).name in keys(env.project_deps), ext_to_parent)))
 
     @debug "precompile: deps collected"
+
+    # An extension effectively depends on another extension if it has all the the
+    # dependencies of that other extension
+    function expand_indirect_dependencies(direct_deps)
+        function visit!(visited, node, all_deps)
+            if node in visited
+                return
+            end
+            push!(visited, node)
+            for dep in get(Set{Base.PkgId}, direct_deps, node)
+                if !(dep in all_deps)
+                    push!(all_deps, dep)
+                    visit!(visited, dep, all_deps)
+                end
+            end
+        end
+
+        indirect_deps = Dict{Base.PkgId, Set{Base.PkgId}}()
+        for package in keys(direct_deps)
+            # Initialize a set to keep track of all dependencies for 'package'
+            all_deps = Set{Base.PkgId}()
+            visited = Set{Base.PkgId}()
+            visit!(visited, package, all_deps)
+            # Update direct_deps with the complete set of dependencies for 'package'
+            indirect_deps[package] = all_deps
+        end
+        return indirect_deps
+    end
+
     # this loop must be run after the full direct_deps map has been populated
-    for (pkg, pkg_exts) in parent_to_exts
-        # find any packages that depend on the extension(s)'s deps and replace those deps in their deps list with the extension(s),
-        # basically injecting the extension into the precompile order in the graph, to avoid race to precompile extensions
-        for (_pkg, deps) in direct_deps # for each manifest dep
-            if !in(_pkg, keys(ext_to_parent)) && pkg in deps # if not an extension and depends on pkg
-                append!(deps, pkg_exts) # add the package extensions to deps
-                filter!(!isequal(pkg), deps) # remove the pkg from deps
+    indirect_deps = expand_indirect_dependencies(direct_deps)
+    for (ext, parent) in ext_to_parent
+        ext_loadable_in_pkg = Dict{Base.PkgId,Bool}()
+        for pkg in keys(direct_deps)
+            pkg_is_extension = in(pkg, keys(ext_to_parent))
+            ext_loadable_in_pkg[pkg] = !pkg_is_extension && pkg != parent && issubset(direct_deps[ext], indirect_deps[pkg])
+        end
+        for (pkg, ext_loadable) in ext_loadable_in_pkg
+            if ext_loadable && !any((dep)->ext_loadable_in_pkg[dep], direct_deps[pkg])
+                # add an edge if the extension is loadable by pkg, and was not loadable in any
+                # of the pkg's dependencies
+                push!(direct_deps[pkg], ext)
             end
         end
     end