Previously the following syntax would fail in the linter with an
error 500:
```
include:
template:
- License-Management.gitlab-ci.yml
- Dependency-Scanning.gitlab-ci.yml
- SAST.gitlab-ci.yml
```
Now the error will call out specifically that the value
is not a string.
Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/66605
This change introduces a support for nesting the includes,
allowing to evaluate them in context of the target,
by properly respecting the relative inclusions and user permissions
of another projects, or templates.
