Make dir_tree() work even when glob= or regexp= argument is supplied

[JoshOBrien]

This PR addresses the issue described in #398. dir_tree() didn't work when a pattern was supplied to regexp= or glob= arguments that get passed on to dir_ls(). That's because the list of file paths returned by those calls to dir_ls() did not include paths of the parent (and grandparent etc.) directories that dir_tree() uses to construct the tree.

The code I've added uses the file paths returned by dir_ls() to recreate the directory paths that'll be needed to build the tree.

To see that this now works, try for example:

dir_tree(system.file(package = "fs"), regexp = "[.]rds$")
## C:/R/Library/fs
## ├── help
## │   ├── aliases.rds
## │   └── paths.rds
## └── Meta
##     ├── features.rds
##     ├── hsearch.rds
##     ├── links.rds
##     ├── nsInfo.rds
##     ├── package.rds
##     └── Rd.rds

dir_tree(system.file(package = "fs"), type = "directory")
## C:/R/Library/fs
## ├── help
## │   └── figures
## ├── html
## ├── libs
## │   └── x64
## ├── Meta
## └── R