partial support for map based selector conversion

[bamarco]

fixes #385

Turns out the selector parsing did not cover cases where maps have selectors.

Given this meta.yml example:

source:
  url: "https://example.com/src-code" # [not win]
  patches:
    - win-specific.patch # [win]

The existing code produces:

source:
  url: "https://example.com/src-code" # [not win]
  patches:
    - sel(win) win-specific.patch

After this pull request you will get:

source:
  - sel(not win)
    :url "https://example.com/src-code"
  patches:
    - sel(win) win-specific.patch

This example is not fully valid yaml either, but I throw a helpful error.

To fully fix the problem you would need to walk every map and make sure that if any of the map entries have a selector then every map entry is converted to a list entry. I think at that point you're halfway to writing a full yaml parser instead of a preprocessor.

A fully fixed version would produce something like:

source:
  - sel(not win)
    :url "https://example.com/src-code"
  - patches:
    - sel(win) win-specific.patch

A real world example:
https://raw.githubusercontent.com/conda-forge/cmdstan-feedstock/main/recipe/meta.yaml

[bamarco]

I examined this a bit further and I think the appropriate result would actually be:

source:
  sel(not win)
    :url "https://example.com/src-code"
  patches:
    - sel(win): win-specific.patch

[bamarco]

I updated the PR to produce valid yaml in more cases and remove the need for the error. It will correctly handle:

source:
  url: "https://windows.example.com/src-code" # [win]
  sha256: 42 # [win]
  url: "https://posix.example.com/src-code" # [not win]
  sha256: 43 # [not win]
  patches:
    - win-specific.patch # [win]

producing:

source:
  sel(win):
    url: "https://windows.example.com/src-code"
    sha256: 42
  sel(not win):
    url: "https://posix.example.com/src-code"
    sha256: 43
  patches:
    - sel(win): win-specific.patch

but will give a duplicate key error for the following:

source:
  url: "https://windows.example.com/src-code" # [win]
  url: "https://posix.example.com/src-code" # [not win]
  sha256: 42 # [win]
  sha256: 43 # [not win]
  patches:
    - win-specific.patch # [win]

because it produces:

source:
  sel(win):
    url: "https://windows.example.com/src-code"
  sel(not win):
    url: "https://posix.example.com/src-code"
  sel(win):
    sha256: 42
  sel(not win):
    sha256: 43
  patches:
    - sel(win): win-specific.patch