How to extend extra-dependencies in an environment?

[plu-bgl]

This question is related to #153, but with the extra-dependencies in case you use another level of templates:

[envs.default]
dependencies = ["dep1", "dep2"]

[envs.dev-base]
extra-dependencies = [ "dep3", "dep4" ]

[envs.experimental]
template = "dev-base"
# here we want to have all dependencies listed in `default` and in `base` and some more

I once used a tool (yocto, but the config files are actually just python files, but the idea can be applied here) and they provided the suffix append and prepend for (some) configuration fields. They were non-destructive and did what they say, the appended or prepended what you defined to the field. This would also have solved #153:

[envs.default]
dependencies = ["dep1", "dep2"]

[envs.dev-base]
dependencies-append = [ "dep3", "dep4" ]

[envs.experimental]
template = "dev-base"
dependencies-append = [ "dep5" ]

So dependencies-append does not overwrite dependencies, but they also do not overwrite dependencies-append defined somewhere else. If you use dependencies = , it overwrites the dependencies defined by its template.