Review behavior: exclusive mapping via install_deps or identity_mapping

[Nour-Mws]

The current behavior (taking into account the PRs currently under review) is that all dependencies not mapped through:

• user-defined mapping
• a local environment

will be matched by exactly one of the following mappings in this order:

• a local env pip-installed from a packages repository if install_deps is true
• identity mapping if identity_mapping is true

If none of these two mappings is turned on, these deps will be unresolved.

In other terms, it's not possible for the resolver to go through more than one mapping in the following set: {local pip-installed venv, identity mapping, unresolved}.

If install_deps is true and pip installing packages unresolved via the first two mappings (local env, user-defined) fails, FawltyDeps will error out and will not attempt to resolve uninstalled deps via other means.

Does this behavior make sense in all cases or can we envisage cases where it should be amended?