sagemathgh-38269: Fix lex_BFS (and co.) for directed graphs

    
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For directed graphs, the semantic of `lex_BFS`, `lex_DFS`, `lex_DOWN`
and `lex_UP` methods are not consistent. In some parts of the code,
directed graphs were converted into undirected graphs before the actual
computation and in some other parts (in particular in the check function
`_is_valid_lex_BFS_order`) directed graphs were explicitly considered as
directed.

In this PR, the following changes are implemented:
- always consider (and convert) directed graphs into undirected graphs
for `lex_*` methods
- do the same conversion in `_is_valid_lex_BFS_order` for consistency
- add the following line in the documentation of the `lex_*` methods:
```py
r"""
   Loops and multiple edges are ignored during the computation of <name
of the method> and
   directed graphs are converted to undirected graphs.
"""
```
- change some doctests: with the previous changes, methods can return a
different (but still valid) order
- set the copy of the graph to immutable

Fixes sagemath#38234

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### ⌛ Dependencies

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URL: sagemath#38269
Reported by: cyrilbouvier
Reviewer(s): cyrilbouvier, David Coudert