Stop Format/STEP.hpp Defining a Global fs Alias (#16102)

Every file that included STEP.hpp, directly or not, got namespace fs = boost::filesystem at global scope, and 29 sources and three headers relied on it without saying so. Headers now spell out boost::filesystem, and each source that uses fs declares the alias itself.

src/slic3r/GUI/Jobs/UpgradeNetworkJob.cpp

@@ -11,6 +11,8 @@
 #include "libslic3r/Utils.hpp"
 #include <exception>
 
+namespace fs = boost::filesystem;
+
 namespace Slic3r {
 namespace GUI {