Feature request: per-source check-sourced directive

[ShamrockLee]

For bugs

• Rule Id (if any, e.g. SC1000):
• My shellcheck version (shellcheck --version or "online"):
• The rule's wiki page does not already cover this (e.g. https://shellcheck.net/wiki/SC2086)
• I tried on https://www.shellcheck.net/ and verified that this is still a problem on the latest commit

For new checks and feature suggestions

• https://www.shellcheck.net/ (i.e. the latest commit) currently gives no useful warnings about this
• I searched through https://github.com/koalaman/shellcheck/issues and didn't find anything related

Here's a snippet or screenshot that shows the problem:

#!/usr/bin/env bash

# I don't want to check this one.
# shellcheck check-sourced=false
source messy.sh 

# I would like to have this checked.
# shellcheck check-sourced=true
source clean.sh

# The rest of the script goes here
echo "Hello, world!"

Here's what shellcheck currently says:

ShellCheck currently doesn't provide such directive.

In temp.sh line 4:
# shellcheck check-sourced=false
             ^-- SC1107 (warning): This directive is unknown. It will be ignored

In temp.sh line 8:
# shellcheck check-sourced=true
             ^-- SC1107 (warning): This directive is unknown. It will be ignored

Here's what I wanted or expected to see:

ShellCheck includes warning from clean.sh but not from messy.sh.

Motivation:

1. Developers sometimes want to include warnings from some sourced files but not from others. This is especially valid if messy.sh cannot be changed from the project scope.
2. This feature provide a workaround for Support for pre-declared "external" globals for sourced scripts to avoid SC2154 warnings. #356, by allowing developers to define a meta-script that sources the pre-sourced files before sourcing the target script to check. The directive proposed here helps exclude warnings from pre-sourced files, while include warnings from the target script.