Search for slnx files when setting solution-relative content root

[kimsey0]

Search for slnx files when setting solution-relative content root

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Fixes #61304

[dotnet-policy-service]

Thanks for your PR, @@kimsey0. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[kimsey0]

This probably isn't a good idea, since it also matches solution filter files (.slnf).

[ninety7]

I can think of a few options:

1. Add another overload for the UseSolutionRelativeContentRoot method that takes in a list of solutionNames and default to ["*.sln", "*.slnx"]. Then we make different calls to Directory.EnumerateFiles and stop on the first return.
2. If the solutionName param is *.sln and the EnumerateFiles call does not return, do an extra lookup for *.slnx.
3. The inverse of 2, and defaulting the parameter to *.slnx.
4. Use your new default of *.sln? and filter solution filter files (.slnf) or any other than .sln, .slnx, if solutionName has the default value.

I think I like option 3 the most.

[kimsey0]

The problem with 1 is that, since the parameters are both optional, there will be a conflict between the overloads. That could maybe be solved with OverloadResolutionPriorityAttribute, but that only works from .NET 9.

2 and 3 are a little weird, since the parameter that you pass in doesn't match the behavior.

[ninety7]

2 and 3 are a bit weird, but even if it's not explicit, sln and slnx are analogous. Maybe do 2 and 3?