GH-46084: [C++] Always use ARROW_VCPKG to detect vcpkg mode (#46467)

### Rationale for this change

We can use `ARROW_DEPENDENCY_SOURCE=VCPKG` to use vcpkg for dependencies. We prepare vcpkg by `cpp/cmake_modules/UseVcpkg.cmake` and it always defines `ARROW_VCPKG`.

The current code base has some ways to detect vcpkg mode:
* `if (ARROW_PACKAGE_KIND STREQUAL "vcpkg")`
* `if (VCPKG_TOOLCHAIN)`

`ARROW_PACKAGE_KIND` is optional. `ARROW_DEPENDENCY_SOURCE=VCPKG` doesn't define `ARROW_PACKAGE_KIND=vcpkg` automatically.

So either `ARROW_VCPKG` or `VCPKG_TOOLCHAIN` is better to detect vcpkg mode.

### What changes are included in this PR?

This change uses `ARROW_VCPKG` for readability. If we accept vcpkg mode without `ARROW_DEPENDENCY_SOURCE=VCPKG`, `VCPKG_TOOLCHAIN` is better than `ARROW_VCPKG`.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Yes.
* GitHub Issue: #46084

Authored-by: Sutou Kouhei <[email protected]>
Signed-off-by: Sutou Kouhei <[email protected]>

Author: kou

cpp/cmake_modules/FindBrotliAlt.cmake

@@ -21,7 +21,7 @@ if(BrotliAlt_FOUND)
   return()
 endif()
 
-if(ARROW_PACKAGE_KIND STREQUAL "vcpkg" OR ARROW_PACKAGE_KIND STREQUAL "conan")
+if(ARROW_VCPKG OR ARROW_PACKAGE_KIND STREQUAL "conan")
   set(find_package_args "")
   if(BrotliAlt_FIND_VERSION)
     list(APPEND find_package_args ${BrotliAlt_FIND_VERSION})
@@ -32,14 +32,14 @@ if(ARROW_PACKAGE_KIND STREQUAL "vcpkg" OR ARROW_PACKAGE_KIND STREQUAL "conan")
   if(BrotliAlt_FIND_REQUIRED)
     list(APPEND find_package_args REQUIRED)
   endif()
-  if(ARROW_PACKAGE_KIND STREQUAL "vcpkg")
+  if(ARROW_VCPKG)
     find_package(BrotliAlt NAMES unofficial-brotli ${find_package_args})
   else()
     find_package(BrotliAlt NAMES brotli ${find_package_args})
   endif()
   set(Brotli_FOUND ${BrotliAlt_FOUND})
   if(BrotliAlt_FOUND)
-    if(ARROW_PACKAGE_KIND STREQUAL "vcpkg")
+    if(ARROW_VCPKG)
       add_library(Brotli::brotlicommon ALIAS unofficial::brotli::brotlicommon)
       add_library(Brotli::brotlienc ALIAS unofficial::brotli::brotlienc)
       add_library(Brotli::brotlidec ALIAS unofficial::brotli::brotlidec)

cpp/cmake_modules/Findutf8proc.cmake

@@ -19,7 +19,7 @@ if(utf8proc_FOUND)
   return()
 endif()
 
-if(ARROW_PACKAGE_KIND STREQUAL "vcpkg" OR VCPKG_TOOLCHAIN)
+if(ARROW_VCPKG)
   set(find_package_args "")
   if(utf8proc_FIND_VERSION)
     list(APPEND find_package_args ${utf8proc_FIND_VERSION})

cpp/cmake_modules/ThirdpartyToolchain.cmake

@@ -2913,7 +2913,7 @@ endmacro()
 
 if(ARROW_WITH_UTF8PROC)
   set(utf8proc_resolve_dependency_args utf8proc PC_PACKAGE_NAMES libutf8proc)
-  if(NOT VCPKG_TOOLCHAIN)
+  if(NOT ARROW_VCPKG)
     # utf8proc in vcpkg doesn't provide version information:
     # https://github.com/microsoft/vcpkg/issues/39176
     list(APPEND utf8proc_resolve_dependency_args REQUIRED_VERSION "2.2.0")