ENH: Create backreferences for default roles

[StefRe]

When adding mini-galleries for API documentation, only the 5 roles func, math, attr, obj and class were taken into consideration. It is, however, common practice to define a default role (e.g. :py:obj:) using the default_role entry in conf.py. This PR enables the creation of backreferences for references without an explicit role if the default role in conf.py is one of the five roles listed above. Note that only the default role definition in conf.py is taken into consideration, i.e. setting the default role using the .. default-role:: directive is not taken into account.

Further, separate titles (Title <link>) and suppressed link creations (!no.link) are handled correctly now.

The project listed in README.rst use the following default roles:

site	default role
Sphinx-Gallery, Nilearn, PyStruct, GIMLi, pyRiemann, Astropy, PySurfer, PyTorch, PyVista, SimPEG, Fury, Optuna, Auto-sklearn, OpenML-Python, TorchIO, Biotite, Apache TVM, Tonic	- (none)
Scikit-learn	literal
scikit-image	autolink
MNE-python, Cartopy, PlasmaPy, Radis	py:obj
Nestle, SunPy, Matplotlib, NetworkX	obj

Projects in the last two rows benefit from this PR as more backreferences/mini-galleries are created, see matplotlib/matplotlib#25595.

This PR comprises two commits:

TST: Refactor test_identify_names2

• split into two tests for implicit and explicit names
• avoid repeated testing of first code_str (code_str and expected were
  extended and the complete code_str tested once more)
• turn into parameterized test for better extensibility

ENH: Allow for default roles when creating backreferences

When adding mini-galleries for API documentation, only the 5 roles
func, math, attr, obj and class were taken into consideration. It is,
however, common practice to define a default role (e.g. :py:obj:) using
the default_role entry in conf.py. This commit enables the creation of
backreferences for references without an explicite role if the default
role in conf.py is one of the five roles listed above.
Please note the only the default role definition in conf.py is taken
into consideration, i.e. setting the default role using the
.. default-role:: directive is not taken into account.

Further, separate titles (Title <link>) and suppressed link creations
(!no.link) are handled correctly now.

[larsoner]

Argh sorry @StefRe I created another conflict for you :(

But +1 in general for this idea, makes sense to me!

[StefRe]

no problem @larsoner , just rebased

[larsoner]

Thanks @StefRe !