bp2build: rename generated files to BUILD

The checked in files are named BUILD.bazel, while bp2build files are
named BUILD.

Test: GENERATE_BAZEL_FILES=true m nothing
Change-Id: If226f581b5075088f774dc4f5f9ee88259821835

bp2build/conversion.go

@@ -62,7 +62,7 @@ func createBuildFiles(buildToTargets map[string]BazelTargets, mode CodegenMode)
 			content += "\n\n"
 		}
 		content += targets.String()
-		files = append(files, newFile(dir, "BUILD.bazel", content))
+		files = append(files, newFile(dir, "BUILD", content))
 	}
 	return files
 }