Improve environment handling

[ZacLN]

julia-vscode/StaticLint.jl#251

This is now in a state in which it's worth a discussion.

The general idea -

• a vscode session has a 'global env', a proper env with a manifest and project file.
• Each root file (i.e. a Document that is not included by another one) get's assigned a subset of the packages in the global env. This assignment is done by checking which workspace folder the root doc is in.
• A workspace folder creates a separate env if it has a project file and is part of the global env.

Example:

Global environment: ~.julia/environments/v1.7
workspace folders ["~/.julia/dev/CSTParser", "~/.julia/dev/StaticLint"]

I have both those projects in the global env.

• "~/.julia/dev/CSTParser/src/CSTParser.jl": Gets all the dependencies specified in "~/.julia/dev/CSTParser/Project.toml" (they're all available from the global env). CSTParser itself is not included.
• "~/.julia/dev/CSTParser/test/runtests.jl": Get's the above packages but also CSTParser and anything included in the extras section of the project file.
• "~/Documents/someotherfile.jl": Gets all packages from the global env (everything in the manifest, not limited to direct dependencies)

EDIT. In terms of the code - there are a lot of lines touched but most of this is just ensuring we're passing the appropriate ExternalEnv to various functions. Substantive changes are to languageserverinstance.jl, textdocument.jl and staticlint.jl

closes julia-vscode/julia-vscode#1248

[pfitzseb]

We'll also need to figure out how this interacts with the env selector widget we have in VSCode, right?

[ZacLN]

Yeah, for the PR as-is the widget controls the global env

[pfitzseb]

Ok that's probably fine for now. I'll try to review this tonight or tomorrow.

[davidanthoff]

Would the idea be that we include this in 1.2, or for later? Should we first finish the cloud downloading stuff, and support for virtual environments? For now I'm not putting that on the list of PRs to include in 1.2, but let me know if you think otherwise.

[pfitzseb]

Ok, I think we should get this in for 1.2 because we need some changes in LanguageServer.jl after merging julia-vscode/StaticLint.jl#251.