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/libslic3r/Print.cpp

@@ -102,6 +102,8 @@
 
 #include <codecvt>
 
+namespace fs = boost::filesystem;
+
 using namespace nlohmann;
 
 // Mark string for localization and translate.