Adding `./src` to path

If you are not using latexmk or for some other reason need too add the ./src/ subfolder to laTeX search-path, here is how to do it:

Using MiKTeX

MiKTeX makes this very easy by adding an option for that. Just add -include-directory=./src to your call to pdflatex.
For building from sub folders, just add a second -include-directory=../src to your call.

Using Texmaker and MiKTeX

As stated above, you just need to extend your Latex-call with the needed include path.
To do this, go to Options → config Texmaker → Commands
change the pdflatex command to:

pdflatex -synctex=1 -interaction=nonstopmode -include-directory=./src -include-directory=../src %.tex

Using VS Code with LaTeX Workshop

In VS Code you can configure your launch to set environment variables. Just edit your settings.json, so that your pdflatex tool contains a env: (add it, if it isn't there yet). If you use the template below, you will also get some additional nice configurations. See also here for some additional configs.

"latex-workshop.latex.tools": [
    {
        "name": "pdflatex",
        "command": "pdflatex",
        "args": [
            "-synctex=1",
            "-interaction=nonstopmode",
            "-file-line-error",
            "-aux-directory=%OUTDIR%/.aux",
            "-output-directory=%OUTDIR%",
            "%DOC%"
        ],
        "env": {
            "TEXINPUTS": "%DIR%/src/;%DIR%/../src/"
        }
    }
]

On Overleaf

As explained here, add the latexmkrc file and put this line into it (which is already part of the latexmkrc provided by the project):

$ENV{'TEXINPUTS'}='./src/:';