Enhancement: Add support for filtering files in Globbing alias

[kcamp]

GlobbingAliases.cs has support for filtering the globbing traversal at the directory level, but no support for filtering files. I have found that support for logic that is more complex than the globbing expression allows has been useful, and have seen gitter traffic for the same issue.

This would be a trivial addition to make at the method alias level. I would propose that two overloads be created --

GetFiles(this ICakeContext context, string pattern, Func<IFile, bool> filePredicate)
 ...

GetFiles(this ICakeContext context, string pattern, Func<IDirectory, bool> predicate, Func<IFile, bool> filePredicate)
...

The former would satisfy the direct need of the feature request, the latter would provide a graceful pairing of current + extended logic.

Assuming that this is something that seems useful and aligned with the intent of the project, the question I would raise is whether it's more appropriate to implement the logic directly in the alias method, similar to ..

// glob the files, and then filter down based on the filePredicate
var files = context.Globber.Match(pattern).OfType<FilePath>()
                            .Where(file => filePredicate(context.FileSystem.GetFile(file)));

or whether it would be more appropriately implemented in the globbing internals -- GlobVisitor, GlobVisitorContext, and so on.

Constraining it to the alias would certainly be less involved, but the latter seems like it would probably be a cleaner (and faster) implementation, particularly in eliminating the extra calls to FileSystem.GetFile(...) - it seems quite apt that it would amount to refactoring the GlobVisitor and GlobVisitorContext so that the logic in FindCandidates would be extended to encompass something like ..

if (includeFiles)
{
    foreach (var file in current.GetFiles("*", option))
    {
        var lastPath = file.Path.Segments.Last();
        if (node.IsMatch(lastPath) && context.ShouldInclude(file))
        ....

Thoughts on whether or not this would be worth pursuing? Any other feedback?

[kcamp]

For what it's worth & work doesn't get duplicated, I've made good progress in extending the globbing to support this and expect to have a PR for it in the next couple of days.

[devlead]

Fixed by #2458