apacheGH-41470: [C++] Reuse deduplication logic for direct registrati…

…on (apache#41466)

### Rationale for this change

As observed in apache#41309 a crossbow job on mac is failing due to duplicate registration of a factory for the file:// scheme

### What changes are included in this PR?

Deduplication of registered filesystem factories is applied to direct registration as well as when merging registries.

### Are these changes tested?

No, we just need to verify that the problematic crossbow job is repaired.

### Are there any user-facing changes?

No

* GitHub Issue: apache#41470

Lead-authored-by: Benjamin Kietzman <[email protected]>
Co-authored-by: David Li <[email protected]>
Signed-off-by: David Li <[email protected]>

cpp/src/arrow/filesystem/localfs_test.cc

@@ -154,15 +154,16 @@ TEST(FileSystemFromUri, RuntimeRegisteredFactory) {
   EXPECT_THAT(FileSystemFromUri("slowfile2:///hey/yo", &path),
               Raises(StatusCode::Invalid));
 
-  EXPECT_THAT(RegisterFileSystemFactory("slowfile2", {SlowFileSystemFactory, "", 0}),
-              Ok());
+  EXPECT_THAT(
+      RegisterFileSystemFactory("slowfile2", {SlowFileSystemFactory, __FILE__, __LINE__}),
+      Ok());
 
   ASSERT_OK_AND_ASSIGN(auto fs, FileSystemFromUri("slowfile2:///hey/yo", &path));
   EXPECT_EQ(path, "/hey/yo");
   EXPECT_EQ(fs->type_name(), "slow");
 
   EXPECT_THAT(
-      RegisterFileSystemFactory("slowfile2", {SlowFileSystemFactory, "", 0}),
+      RegisterFileSystemFactory("slowfile2", {SlowFileSystemFactory, __FILE__, __LINE__}),
       Raises(StatusCode::KeyError,
              testing::HasSubstr("Attempted to register factory for scheme 'slowfile2' "
                                 "but that scheme is already registered")));