Use std::filesystem functions in more places

This makes for shorter and more portable code.

The only tricky part is catching exceptions: I just searched for near by
`catch (Error &)` or `catch (SysError &)` and adjusted them to `catch
(std::filesystem::filesystem_error &)` according to my human judgement.

Good for windows portability; will help @siddhantk232 with his GSOC
project.

src/libstore/profiles.cc

@@ -338,6 +338,8 @@ Path getDefaultProfile()
         return absPath(readLink(profileLink), dirOf(profileLink));
     } catch (Error &) {
         return profileLink;
+    } catch (std::filesystem::filesystem_error &) {
+        return profileLink;
     }
 }