ClickOnceSigner crashes if other .manifest files present

We have a folder with multiple Office add-ins, each with a `vsto` and `manifest` file.

Attempting to sign one `vsto`, e.g. `Hello.World.OutlookAddIn.vsto` throws

```
fail: Sign.Core.ISigner[0]
Sequence contains more than one matching element
System.InvalidOperationException: Sequence contains more than one matching element
at System.Linq.ThrowHelper.ThrowMoreThanOneMatchException()
at System.Linq.Enumerable.TryGetSingle[TSource](IEnumerable`1 source, Func`2 predicate, Boolean& found)
at System.Linq.Enumerable.SingleOrDefault[TSource](IEnumerable`1 source, Func`2 predicate)
at Sign.Core.ClickOnceSigner.<>c__DisplayClass9_0.<b__0>d.MoveNext() in /_/src/Sign.Core/DataFormatSigners/ClickOnceSigner.cs:line 130
--- End of stack trace from previous location ---
at System.Threading.Tasks.Parallel.<>c__53`1.<b__53_0>d.MoveNext()
--- End of stack trace from previous location ---
at Sign.Core.ClickOnceSigner.SignAsync(IEnumerable`1 files, SignOptions options) in /_/src/Sign.Core/DataFormatSigners/ClickOnceSigner.cs:line 82
at Sign.Core.AggregatingSigner.SignAsync(IEnumerable`1 files, SignOptions options) in /_/src/Sign.Core/DataFormatSigners/AggregatingSigner.cs:line 204
at Sign.Core.Signer.<>c__DisplayClass3_0.<b__0>d.MoveNext() in /_/src/Sign.Core/Signer.cs:line 155
--- End of stack trace from previous location ---
at System.Threading.Tasks.Parallel.<>c__53`1.<b__53_0>d.MoveNext()
--- End of stack trace from previous location ---
at Sign.Core.Signer.SignAsync(IReadOnlyList`1 inputFiles, String outputFile, FileInfo fileList, DirectoryInfo baseDirectory, String applicationName, String publisherName, String description, Uri descriptionUrl, Uri timestampUrl, Int32 maxConcurrency, HashAlgorithmName fileHashAlgorithm, HashAlgorithmName timestampHashAlgorithm) in /_/src/Sign.Core/Signer.cs:line 84
```

The search for the `.manifest` should probably include the rest of the file name, e.g. `Hello.World.OutlookAddIn.*.manifest`.