Fix chart_schema.yaml import-values rule

[forquare]

What this PR does / why we need it:

The existing rule for validating dependency import-values worked well for either of these two cases:

1. Importing exports

dependencies:
- name: dep1
  version: 0.14.1
  repository: oci://myrepo
  import-values:
    - data

2. Importing child values into the parent

dependencies:
- name: dep1
  version: 0.14.1
  repository: oci://myrepo
  import-values:
    - child: default.data
      parent: myimports

However, it failed on the following valid usecase:

dependencies:
- name: dep1
  version: 0.14.1
  repository: oci://myrepo
  import-values:
    - data
    - child: default.data
      parent: myimports

Giving the following error:

dependencies.0.import-values.1: '{'parent': 'myimports', 'child': 'default.data'}' is not a str.
dependencies.0.import-values.0 : 'data' is not a map

This is because the Yamale is validating on a list of strings or a list of import-value, where a list containing both is also valid.

This commit changes the rule to allow both methods to be in the import-values list.