Separating functions folder into build folder and src folder

[peter-wd-1]

This is one of two requests in #276

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Hi,
I have lambda functions that have their own dependency and so need to use netlify-lambda install and It only detects build folder(relatively designated functions folder becomes build folder because there is no way to specify it as src folder). If I want to separate src folder and build folder for functions, I need to specify netlify-lambda install and I can't install with just one line of the script. Instead, I need to specify all the lambda functions folder with the install command. I think this is very handful way of installing functions. I really can't think of use case that netlify-lambda install would not need to check all lambda folders and install each dependency(Users love convenience as we all know).

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In netlify.toml, I would like to designate two different folders for src and build like below:

functions-src = "./src/functions"
functions-build = "./functions-build"

With just one line of code for installing dependency of all functions in package.json.

"scripts":{
  "postinstall": "netlify-lambda install",
}