Exchange values between task and deps using dotenv files.

[trulede]

This PR allows the exchange of variables between tasks, and depend tasks, using the dotenv mechanism.

If a task has deps and dotenv then the dotenv files are reloaded which makes them available in the environment of subsequent task commands.

Using this method the task does not need to be recompiled, which avoids complexities which might occur if templated variables in a task would change.

Fixes #1381 (and many others)
Fixes #1575
Fixes #2385

Alternative to #1746 (improves upon)
Related to #1627 (alternative to)

In general, works as you might expect:

version: '3'
tasks:
  gen:
    cmds:
      - echo "BAR=bar" > .genenv  
  default:
    dotenv:
      - .genenv
    deps:
      - gen
    cmds:
      - echo "BAR=$BAR"