Analysis engine should not analyze entire project

[DonJayamanne]

I don't think the analysis engine should analyze the entire workspace folder and report syntax errors.
Currently that's the default behaviour:

Problems:

• One might have tests files with errors in them for specific test cases (Analysis engine should not provide linter messages if linting is disabled #1809 (comment))
• There might be some scripts in a folder that are specific to python 2 or python 3 only, and the interpreter selected is the opposite (3 or 2, vice versa).

As a result we have a lot of noise and false positives being reported.
If the code base is quite large, then valid errors/messages returned by linters get buried in the noise.

I'm flagging this as a bug as it produces too many false positives, & would be too confusing for developers moving from Jedi to Analysis engine.

Solution:

• Provide setting to turn on/off the ability to scan entire workspace for syntax errors.

@qubitron @brettcannon cc

[MikhailArkhipov]

There is already some filtering for virtual environments residing in the workspace. Basically we just need some logic to determine which folders must be excluded. It is simpler in VS that has 'white list' model in most projects.

[patrys]

@MikhailArkhipov When I tested yesterday most errors reported were in fact from packages installed in my virtualenv (${workspaceRoot}/.direnv/python-3.6.5) so I don't think that filter works.

[brettcannon]

Having a path component of site-packages is the best solution I can think of off the top of my head.

[patrys]

I think the extensions is already aware of where the venv is? The other candidate would be node_modules as there are several Node packages using gyp as a build system.

[MikhailArkhipov]

PR is up microsoft/PTVS#4372