Rework project discovery

[jmgomez]

In order to avoid spawning unnecessary nimsuggest processes we must improve the way the lang server handles files.

For instance, when the user opens a file (i.e. navigates to a type) that is part of the nimble requires the langserver should not spawn a new nimsuggest process but to reuse the existing one. The discovery process should also take into account things like opening a file in an arbitrary package. So instead of using the current file as a program itself, it should try to find the nimble project where it belongs (this may require changes in nimble). Changes in files should also be taking into consideration, for instance if the nimble entry point import module b, the module b nimsuggest's process must be closed and use the nimble one instead.

In order to do the above, we must first document via tests what's currently supported.

[jmgomez]

When opening testproject/submodule.nim which is not imported by testproject.nim the langserver gets all wrong. It sets as main project testproject.nim which effectively left out of IDE support to the submodule.nim file.
What it should instead is to create a nimsuggest instance for submodule.nim if it is not part of the testproject.nim which isnt because it's not being imported. If it were imported, the behaviour would be correct.