GH-46388: [C++] Check Snappy::snappy{,-static} in FindSnappyAlt.cmake (#46389)

### Rationale for this change

Other projects' `FindSnappy.cmake` may create `Snappy::snappy{,-static}` CMake targets. If they are created, our `FindSnappyAlt.cmake` will be failed.

### What changes are included in this PR?

Check whether `Snappy::snappy{,-static}` are already defined to avoid conflicting with other projects' `FindSnappy.cmake`.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Yes.
* GitHub Issue: #46388

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>

Author: kou

cpp/cmake_modules/FindSnappyAlt.cmake

@@ -19,6 +19,18 @@ if(SnappyAlt_FOUND)
   return()
 endif()
 
+if(ARROW_SNAPPY_USE_SHARED)
+  if(TARGET Snappy::snappy)
+    set(Snappy_TARGET Snappy::snappy)
+    set(SnappyAlt_FOUND TRUE)
+    return()
+  elseif(TARGET Snappy::snappy-static)
+    set(Snappy_TARGET Snappy::snappy-static)
+    set(SnappyAlt_FOUND TRUE)
+    return()
+  endif()
+endif()
+
 set(find_package_args)
 if(SnappyAlt_FIND_VERSION)
   list(APPEND find_package_args ${SnappyAlt_FIND_VERSION})