linkage_checker: check variable references with dlopen

The linkage check currently does nothing to check the validity of
variable-referenced libraries (prefixed with an `@`).

We could rectify that by mimicking the dynamic linker in looking up the
variable-referenced library, but this could get quite complicated.
Instead, let's let the linker do the hard work by checking if we can
`dlopen` libraries and bundles that contain variable linkage. The
`dlopen` will fail if the linker cannot resolve the variable reference.

There are at least two disadvantages to this approach relative to the
alternative suggested above:
1. This doesn't work for binary executables.
2. This doesn't identify which variable references are broken.

It's still better than not checking them at all, which is what we do
currently, and saves us from having to carry around code that parses and
verifies variable references directly.

Library/Homebrew/linkage_checker.rb

@@ -32,6 +32,7 @@ class LinkageChecker
     @unnecessary_deps = []
     @unwanted_system_dylibs = []
     @version_conflict_deps = []
+    @broken_variable_dylibs = []
 
     check_dylibs(rebuild_cache: rebuild_cache)
   end
@@ -46,6 +47,7 @@ class LinkageChecker
     display_items "Undeclared dependencies with linkage", @undeclared_deps
     display_items "Dependencies with no linkage", @unnecessary_deps
     display_items "Unwanted system libraries", @unwanted_system_dylibs
+    display_items "Libraries with broken variable references", @broken_variable_dylibs
   end
 
   def display_reverse_output
@@ -68,11 +70,12 @@ class LinkageChecker
     display_items "Broken dependencies", @broken_deps, puts_output: puts_output
     display_items "Unwanted system libraries", @unwanted_system_dylibs, puts_output: puts_output
     display_items "Conflicting libraries", @version_conflict_deps, puts_output: puts_output
+    display_items "Libraries with broken variable references", @broken_variable_dylibs, puts_output: puts_output
   end
 
   sig { returns(T::Boolean) }
   def broken_library_linkage?
-    issues = [@broken_deps, @unwanted_system_dylibs, @version_conflict_deps]
+    issues = [@broken_deps, @unwanted_system_dylibs, @version_conflict_deps, @broken_variable_dylibs]
     [issues, unexpected_broken_dylibs, unexpected_present_dylibs].flatten.any?(&:present?)
   end
 
@@ -148,6 +151,7 @@ class LinkageChecker
     end
 
     checked_dylibs = Set.new
+    dlopened_if_needed_files = Set.new
 
     keg_files_dylibs.each do |file, dylibs|
       dylibs.each do |dylib|
@@ -159,6 +163,15 @@ class LinkageChecker
 
         if dylib.start_with? "@"
           @variable_dylibs << dylib
+
+          if dlopened_if_needed_files.add?(file)
+            file = Pathname.new(file)
+            next if file.binary_executable?
+            next if dylib_found_via_dlopen(file.dylib_id)
+
+            @broken_variable_dylibs << file.dylib_id
+          end
+
           next
         end