parse: parse target correctly for generated stages

[skshetry]

Even the title says correctly, this is done to support stages that was generated using a complex data, which is not officially supported.

eg:

stages:
  build:
    foreach:
      - level: 15
      - level: 25
      - level: 35
    do:
      cmd: echo ${item.level}

Here, the list has dict inside which is something that's not supported officially. This will generate stages having name similar to the
build@{'level': 15}, which works, but the dvc repro <stage> fails as the target parser looks for : to differentiate between file path and the stage name (eg: we support running single stage via dvc repro ../dvc.yaml:build2).

Though, I have to say, the parsing is getting complex. :(

Fixes #4957

• ❗ I have followed the Contributing to DVC checklist.

• 📖 If this PR requires documentation updates, I have created a separate PR (or issue, at least) in dvc.org and linked it here.

Thank you for the contribution - we'll try to review it as soon as possible. 🙏

[jorgeorpinel]

Hey @skshetry your PR descriptions are pretty good, they explain what you're developing. Do you have a wiki or something that concentrates all of these explanations for the vars and loops in dvc.yaml? We need to start drafting the docs thig month AFAIK. Thanks

[skshetry]

@jorgeorpinel, all of these are documented in the https://github.com/iterative/dvc/wiki/Parametrization.
All, except set, are official features.

[jorgeorpinel]

Purrrfect 🐱