Add support for custom object module

[ryuukk]

Fixes #688

[ryuukk]

by default object.d is implicitly imported in every modules

you can override this module by creating an object.d in your project, that's how one create a custom runtime: https://github.com/adamdruppe/webassembly/blob/master/arsd-webassembly/object.d

the current DCD behavior is to just pick the first one to come, if you still use the druntime, but still want to override object.d, then DCD will always pick the one from druntime, since import path for druntime is always provided first

i decided against filtering, because of issues like this: https://github.com/Pure-D/serve-d/issues/260

so it's best to have a safe behavior, custom object.d is a niche use case anyways, so have both code completion for default object.d and the custom one is better than having just the default one

[WebFreak001]

I agree custom object.d is a niche use-case, but we want to support it properly anyway, because there are quite a few valid use-cases which also have been used in the community. (e.g. webassembly druntime, lwdr, etc)

So instead of duplicating a lot of the import lookup code just for the implicit import object, which should not be any special, we should fix the ordering of the import paths. The same issue applies to all of phobos as well: if you define a custom std/stdio.d, it should override phobos, and does so with dmd, but DCD will still auto-complete from phobos instead. And the merging thing doesn't work that well there.

[ryuukk]

I think this PR should be revived, i work exclusively with a custom runtime (wasm), and not having completions for my type is kinda annoying

[WebFreak001]

I reevaluated this and it looks like what DCD is doing already is correct and your problem is DCD misconfiguration from the IDE. However only being able to add import paths to the end, where they have less precedence seems to make this unnecessarily complicated. In the compiler the first specified import also has highest precedence, but all the built-in imports from the compiler are always appended to the end and not to the start, like what DCD does.

Since DCD does not have any notation of what is a built-in import from the compiler and relies on the IDE for all imports (or external user-configuration that probably shouldn't be loaded in an IDE environment), the order is completely dependent on the IDE and the API that DCD currently exposes to modify them is quite cumbersome for IDE developers.

What an IDE should do to support this properly is:

• maintain the current list of imports
• on change:
  • remove all the imports after where a new import would be inserted
  • add the new imports
  • re-add all removed imports again

It'd probably be a good idea to make a simpler DCD command for that for IDEs, e.g. --allImports which replaces the internal import list completely.