Fix stubname equal suffix in wide to long function

[tqa236]

• closes BUG: pd.wide_to_long can't handle stubname equal to suffix #46939(Replace xxxx with the GitHub issue number)
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature. (Will add after review)

As pandas allows an index in a multiindex and a column to have the same name, I think it's reasonable to allow stubname equal suffix.

However, I think the root cause is in the melt function. It's stated in the docstring that

    value_name : scalar, default 'value'
        Name to use for the 'value' column, can't be an existing column label.

But if value_name == var_name, illogical result is returned. Example:

df = pd.DataFrame(
    {
        "A": {0: "a", 1: "b", 2: "c"},
        "B": {0: 1, 1: 3, 2: 5},
        "C": {0: 2, 1: 4, 2: 6},
    }
)
pd.melt(
    df,
    id_vars=["A"],
    value_vars=["B"],
    var_name="myVarname",
    value_name="myVarname",
)

Result:

I think it's more reasonable to also raise an error if value_name == var_name in melt

[tqa236]

Hi @mroeschke, can you take a look at this PR, please? It should be ready for review.