…get its anchor

This change introduces a new attribute on collectors: `fallback_config`.
It is used by the `Handlers.get_anchor` method when collecting an object
to find its anchor. It allows to pass specific options to the collector
to avoid collecting the object members or, for example,
parsing Python docstrings, as it is not useful,
since we only care about the anchor.

Issue #329: #329
PR #330: #330

Since an object can have aliases (different identifiers
leading to it), and since users sometimes want to
render an object using one of its aliases instead of its
canonical identifier, we make sure to register every
identifier associated to an object so that autorefs
can find it when fixing cross-references.

Issue mkdocstrings/autorefs#11: mkdocstrings/autorefs#11
PR #350: #350

Issue #205: #205
Issue #324: #324
PR #331: #331

Issue #341: #341
PR #342: #342