include generated files in the checksum method

[superg]

This PR includes generated files into the calculated checksum to trigger task only when needed.

Namely, that implements:
#2181
and closes:
#2301 (original bug report for timestamp method but this PR will allow to use default checksum method and preserves original timestamp method behavior)

Example Taskfile.yml:

version: '3'

tasks:
  default:
    sources:
      - input/*.in
    generates:
      - output/*.out
    cmds:
      - mkdir -p output
      - cp input/one.in output/one.out
      - cp input/two.in output/two.out

• initial task run will generate one.out and two.out
• subsequent task run will report task is up to date
• rename of any *.out file and running task will generate the files again
• removal of any *.out file and running task will generate the files again