Redesign project file resolver

### What needs to get done

`_resolve_project_path()`

1. should find a project file in multiple locations.
2. not fail if a directory is actually a file

### Why it needs to get done

1. Applications have to hack around this (https://github.com/canonical/snapcraft/blob/5317dbebac1c953192ff38e4c6fddd294f979cb1/snapcraft/application.py#L154-L165)
2. craft-application raises an internal error (https://github.com/canonical/snapcraft/issues/5167)

Start at `_resolve_project_path()` and read the linked snapcraft workaround in `application.py`, along with the referenced craft-application issue. Done means the resolver finds a project file in multiple locations and does not fail when a directory path is actually a file; the issue does not name a test file to run.

Written by the indexing model from the issue text.