Support artman config without REPOROOT about artman

[lukesneeringer]

I took a different pass at this with local_paths, which allows each repository to either be derived from reporoot or explicitly defined. I assert it probably solves for this problem.

Note that in the remote pipeline, we will just check out the applicable repos. I think that is orthogonal to this issue, though.

All paths except the old gapic_code_dir (which used to be final_repo_dir) no longer derive from ${REPOROOT}. I actually intend to scrap gapic_code_dir completely and just make it an automatically determined temporary directory (/tmp/{uuid.uuid4()[0:8]}/), since it is no longer used to determine code's final staging location (except when using --publish=noop, which we do not even document).

[geigerj]

@lukesneeringer For context, this issue was originally raised by some of our users who were running the remote pipeline with protos that were not yet checked into the applicable repos; we supported a phase in which the users specified a local directory that was then uploaded and merged into a clone of the googleapis repo. This caused quite a lot of confusion from our users about what files were coming from where.

[lukesneeringer]

I see. My changes do not really make that particular problem much better (although they certainly do not make it any worse). Really the issue is not with reporoot, it is with googleapis.

The ultimate solution to this problem is probably to infer basically everything that is not in the artman or GAPIC YAML files; we do not really need to outright specify lang/common.yaml every time; it would be entirely okay to simply make everything in that file settings that can be specified in a user YAML but which have these defaults otherwise.

This clearly needs more design work. Leaving this issue in place.

[ethanbao]

This is no longer required under new artman syntax.