Make deps respect --project-dir

[beckjake]

resolves #2519
resolves #2541

Description

The problem was actually the opposite of what I thought: dbt deps does the wrong thing if you give it --project-dir, and puts files relative to the current directory. But it does look in the --project-dir to find packages.yml, so it just downloads them to the wrong directory.
Subsequent dbt commands that do respect --project-dir then can't find their dependencies, because they've moved into the project directory and away from the starting directory.

Checklist

• I have signed the CLA
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• I have updated the CHANGELOG.md and added information about my change to the "dbt next" section.

[beckjake]

I do not understand how this change causes that test to fail in that way. This test passes locally (both in isolation and in parallel) but consistently fails the exact same way in circle. Nothing about this PR should change that test's behavior and it makes no sense to me that the schemas could already exist.

[beckjake]

I made a bunch of silly test changes to drop more schemas and try to avoid collisions, which seems to have fixed the issue

[drewbanin]

I'm going to take some more time to comb through the diff in this PR, but I did want to call something out I noticed while debugging:

I think we've sort of flip-flopped on regressions for this feature over the last couple of versions of dbt:

version	command	status
0.16.0	dbt deps --project-dir projectdir	success
0.16.0	dbt run --project-dir projectdir	success
0.16.1	dbt deps --project-dir projectdir	failed
0.16.1	dbt run --project-dir projectdir	success
0.17.0	dbt deps --project-dir projectdir	installed packages to wrong dir
0.17.0	dbt run --project-dir projectdir	failed
this branch	dbt deps --project-dir projectdir	failed
this branch	dbt run --project-dir projectdir	failed

Each of the invocations marked failed errored out immediately with:

Running with dbt=<version>
Encountered an error while reading the project:
  ERROR: Runtime Error
  no dbt_project.yml found at expected path projectdir/dbt_project.yml
Encountered an error:
Runtime Error
  Could not run dbt

So, the key takeaway is that I'm seeing --project-dir is not working correctly on this branch, but there also might be some broader issues for us to investigate here than dbt_modules simply being written to the wrong dir.

I made a loom repro'ing this error in this branch: https://www.loom.com/share/54077f3fc52344d88ccfddc0d6e5ee19

[beckjake]

@drewbanin You have to give the full path to your project directory.

[kwigley]

Just one comment, everything else looks good to me!

[kwigley]

A little concern with possible behavior this line might introduce. I think the assumption here is that load_dependencies will be called after moving to the root project dir. I think it is fine, will defer to you!

[drewbanin]

LGTM! Thanks for hitting #2541 here also - that's a great change