Add $CRYSTAL_INTERPRETER_LOADER_INFO to show loaded libraries (#12221)

With this change, you can ask the interpreter to print a list of loaded libraries, similar to what you would get with `ldd` for a compiled progam.
This feature is opt-in and can be enabled by setting the environment variable `CRYSTAL_INTERPRETER_LOADER_INFO` to a non-empty value.

src/compiler/crystal/interpreter/context.cr

@@ -370,13 +370,24 @@ class Crystal::Repl::Context
     args.delete("-lgc")
 
     Crystal::Loader.parse(args).tap do |loader|
+      if ENV["CRYSTAL_INTERPRETER_LOADER_INFO"]?.presence
+        STDERR.puts "Crystal::Loader loaded libraries:"
+        loader.loaded_libraries.each do |path|
+          STDERR.puts "      #{path}"
+        end
+      end
+
       # FIXME: Part 2: This is a workaround for initial integration of the interpreter:
       # We append a handle to the current executable (i.e. the compiler program)
       # to the loader's handle list. This gives the loader access to all the symbols in the compiler program,
       # including those from statically linked libraries like libgc.
       # This probably won't work for a fully statically linked compiler.
       # But `Crystal::Loader` currently doesn't support that anyways.
       loader.load_current_program_handle
+
+      if ENV["CRYSTAL_INTERPRETER_LOADER_INFO"]?.presence
+        STDERR.puts "      current program handle"
+      end
     end
   }
 

src/compiler/crystal/loader.cr

@@ -45,6 +45,7 @@ class Crystal::Loader
   end
 
   getter search_paths : Array(String)
+  getter loaded_libraries = [] of String
 
   def initialize(@search_paths : Array(String))
     @handles = [] of Handle

src/compiler/crystal/loader/msvc.cr

@@ -74,7 +74,9 @@ class Crystal::Loader
       # (https://docs.microsoft.com/en-us/windows/win32/dlls/dynamic-link-library-search-order)
       handle = open_library(dll)
       return false unless handle
+
       @handles << handle
+      @loaded_libraries << dll
     end
 
     true

src/compiler/crystal/loader/unix.cr

@@ -89,6 +89,7 @@ class Crystal::Loader
     return false unless handle
 
     @handles << handle
+    @loaded_libraries << path.to_s
     true
   end