Add collection methods to RunfilesSubject

While asserting some runfiles, I needed to do something like:

```
target.runfiles().contains_at_least([
,

])
```

Unfortunately, there isn't a great way to do this.

At the least, I think these convenience methods need to be added:

* `.paths() -> CollectionSubject[str]`: returns all the runfiles paths that are represented.
* `.path_map() -> DictSubject[str, File|None]`: returns a mapping of path to File (or None, in the case of `.empty_filenames`). Unfortunately, this is, potentially, a lossy operation -- a symlink could have the same path as a regular file entry. So we would want to match Bazel's behavior for how it handles such cases as best we can.

And these two lower-level accessors need to be added:
* `.files() -> DepsetFileSubject`: returns the `Runfiles.file` value as a subject
* `.empty_filenames() -> CollectionSubject[str]`: Returns `Runfiles.empty_filenames` as a subject
* `.symlinks() -> DepsetSubject[SymlinkEntry]`: Returns `Runfiles.symlinks` as a subject
* `.root_symlinks() -> DepsetSubject[SymlinKEntry]`: Returns `Runfiles.root_symlinks` as a subject

I'm not sure how to best directly expose other methods because a runfiles object is a heterogenous collection -- Files, symlink entries, and str (empty file names). I think matching on the str paths is about the best we can do -- its the common denominator. For a more specific check, the other accessors can be used (e.g. to check that something is a symlink).