Fix for "treat output as directory" for multiple input files

[spilkercompra]

These comments in Signer.cs should be true:

// Special case if there's only one input file and the output has a value, treat it as a file [...] // See if it has a file extension and if not, treat as a directory and use the input file name [...] // if the output is specified, treat it as a directory, if not, overwrite the current file

[clairernovotny]

LGTM -- @dtivel?

[spilkercompra]

@dotnet-policy-service agree