pw_build: Switch to pw_* target types

This change replaces the implicit overrides of the default GN target
types in BUILDCONFIG.gn with explicit pw_* versions. This is done to
allow Pigweed to build within larger GN projects without modifying their
BUILDCONFIG files.

Change-Id: I1d1e41d5e9bde8fe076f7031f3c2b98b9630ff70
diff --git a/pw_checksum/BUILD.gn b/pw_checksum/BUILD.gn
index 0a7e169..e17803b 100644
--- a/pw_checksum/BUILD.gn
+++ b/pw_checksum/BUILD.gn
@@ -12,6 +12,7 @@
 # License for the specific language governing permissions and limitations under
 # the License.
 
+import("$dir_pw_build/target_types.gni")
 import("$dir_pw_docgen/docs.gni")
 import("$dir_pw_unit_test/test.gni")
 
@@ -19,7 +20,7 @@
   include_dirs = [ "public" ]
 }
 
-source_set("pw_checksum") {
+pw_source_set("pw_checksum") {
   public_configs = [ ":default_config" ]
   public = [ "public/pw_checksum/ccitt_crc16.h" ]
   sources = [ "ccitt_crc16.cc" ] + public