Drop dependency on FileUtils

Fix: #404

But also it's annoying to have delayed `require` like this one.

CHANGELOG.md

@@ -1,5 +1,7 @@
 # Unreleased
 
+* Drop dependency on `fileutils`.
+
 * Better respect `Kernel#require` duck typing. While it almost never comes up in practice, `Kernel#require`
   follow a fairly intricate duck-typing protocol on its argument implemented as `rb_get_path(VALUE)` in MRI.
   So when applicable we bind `rb_get_path` and use it for improved compatibility. See #396 and #406.

lib/bootsnap/load_path_cache/store.rb

@@ -89,19 +89,17 @@ def load_data
       end
 
       def dump_data
-        require "fileutils" unless defined? FileUtils
-
         # Change contents atomically so other processes can't get invalid
         # caches if they read at an inopportune time.
         tmp = "#{@store_path}.#{Process.pid}.#{(rand * 100_000).to_i}.tmp"
-        FileUtils.mkpath(File.dirname(tmp))
+        mkdir_p(File.dirname(tmp))
         exclusive_write = File::Constants::CREAT | File::Constants::EXCL | File::Constants::WRONLY
         # `encoding:` looks redundant wrt `binwrite`, but necessary on windows
         # because binary is part of mode.
         File.open(tmp, mode: exclusive_write, encoding: Encoding::BINARY) do |io|
-          MessagePack.dump(@data, io, freeze: true)
+          MessagePack.dump(@data, io)
         end
-        FileUtils.mv(tmp, @store_path)
+        File.rename(tmp, @store_path)
       rescue Errno::EEXIST
         retry
       rescue SystemCallError
@@ -110,6 +108,21 @@ def dump_data
       def default_data
         {VERSION_KEY => CURRENT_VERSION}
       end
+
+      def mkdir_p(path)
+        stack = []
+        until File.directory?(path)
+          stack.push path
+          path = File.dirname(path)
+        end
+        stack.reverse_each do |dir|
+          begin
+            Dir.mkdir(dir)
+          rescue SystemCallError
+            raise unless File.directory?(dir)
+          end
+        end
+      end
     end
   end
 end

test/load_path_cache/store_test.rb

@@ -71,7 +71,7 @@ def test_retry_on_collision
 
         MessagePack.expects(:dump).in_sequence(retries).raises(Errno::EEXIST.new("File exists @ rb_sysopen"))
         MessagePack.expects(:dump).in_sequence(retries).returns(1)
-        FileUtils.expects(:mv).in_sequence(retries)
+        File.expects(:rename).in_sequence(retries)
 
         store.transaction { store.set("a", 1) }
       end