Support flat git repositories

[timothyb89]

Flat git repositories should be supported (as per https://github.com/monasca/pr-bot/blob/afdf55ab7fbec2af367144f020b86900d6646cd4/lib/repository/git.js#L305).

'flat' in this case would mean a single module in a repository, presumably not in a subdirectory. Most single-project repositories (not like monasca-docker) are built like this, including the pr-bot repo itself.

[timothyb89]

The repository scanners added in 838c7f9 solve part of the issue. Now it's trivial to find a module initially (flat or otherwise), but we'll need some sort of hinting mechanism when going the other direction, i.e. how can we take a module name and find it on the filesystem? So far the module name directly corresponds to filesystem path (even landscaper, albeit in a convoluted way), but we can't depend on that with flat repositories.

Some ideas:

• mark the whole repository as flat (new property of GitRepository)
  • modulePath() checks flat flag and returns localPath when set, otherwise falls back to current resolution logic
  • however, this partially breaks support for many coexisting modules of different types in a repository, which we just gained with the module scanner
• add new optional field to Module to hint at the path
  • fall back to current logic if field is unset
  • downside: adds git-specific metadata to Module
  • should all modules be given a path hint, or just those that fail with current logic?

[timothyb89]

Partial support was added in https://github.com/monasca/pr-bot/commit/4bd94e5a808f52a39399c2a8bbb5d96018fa0e05 to support docker-compose plugins. A new path field was added to Module and modulePath() now checks that value if set. Check plugins need to specifically support root-level modules by setting path to ., currently only the docker-compose plugin implements this.