go: cannot find main module, but found .git config to create a module there, run: go mod

This problem occurred in a monorepo in VSCode. The repo has many services placed in isolated directories with their respective go.mod files.

I realized from various sources that the solution has two parts:

1. Move the mono repo under $GOPATH/src. This solves many problems including ctags code navigation.
2. The ~/.zshrc or ~/.bashrc must be as follows:

export GOPATH=$HOME/go/ 
export GOROOT=/usr/local/go  
export PATH="$GOROOT/bin:$PATH" 
export PATH="$PATH:$GOPATH/bin"

3. Do not add any other variable that affects gopls such as GO111MODULE=on.