use silence for avoiding reoccuring warnings

better solution would be to avoid / in usepackage:
https://tex.stackexchange.com/questions/31925/install-package-in-subfolder/31950#31950
but I didn't find how to set the env variable TEXINPUTS in .latexmkrc

tex/header/preamble.tex

@@ -8,6 +8,12 @@
 %   \RedeclareSectionCommand
 %     [beforeskip=-1.5ex plus -.1ex minus -.1ex,
 %      afterskip=.5ex plus .1ex minus .1ex]{subsection}
+
+% cannot happen in packages since it has to work during package loading:
+% (and it is really not a fault of the packages but the calling)
+\usepackage{silence}                  % Package that lets ignore warnings and errors for using packages in subdirectories
+\WarningFilter{latex}{You have requested package `.maindir/tex/header/}
+
 \usepackage[tikz]{.maindir/tex/header/stdmath} % , scriptLetters
 
 \setmainfont{Latin Modern Roman}