Use std::filesystem instead of std::experimental

std::filesystem is fully supported in c++17, no need to keep
the experimental around anymore.

Note that the path append behavior changed, so appending an
absolute path replaces the path instead of appending, therefore
need to use .relative_path().

Tested: Did a code update on witherspoon and verified the sync
        path was correct.

Change-Id: Iffe42cf0ebd5537920c8fdbf9edcec640ab9c724
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
diff --git a/image_manager.cpp b/image_manager.cpp
index e132465..226199c 100644
--- a/image_manager.cpp
+++ b/image_manager.cpp
@@ -14,7 +14,7 @@
 #include <algorithm>
 #include <cstring>
 #include <elog-errors.hpp>
-#include <experimental/filesystem>
+#include <filesystem>
 #include <phosphor-logging/elog.hpp>
 #include <phosphor-logging/log.hpp>
 #include <string>
@@ -33,7 +33,7 @@
 using ManifestFail = Software::Image::ManifestFileFailure;
 using UnTarFail = Software::Image::UnTarFailure;
 using InternalFail = Software::Image::InternalFailure;
-namespace fs = std::experimental::filesystem;
+namespace fs = std::filesystem;
 
 struct RemovablePath
 {