Add options to constrain and broaden files searched

[maw]

I added some code to cycle between search scopes:

• Searching the currently specified directory. This is how deadgrep has always worked.
• Limit the search to files that correspond to open buffers in the currently specified directory.
• Expand the search to all files that correspond to open buffers, both in and out of the currently specified directory.

I think this is cool. But here are some problems (that I am willing to address)

• Some of the new code is probably in the wrong place in deadgrep.el
• What this new stuff means probably isn't obvious enough in the UI or the documentation
• My set theory is rusty so I forget what this should be called, but there's a fourth scope that I didn't implement: all files in currently selected directory plus all (other) open files.
• If you're in the "open buffers" scope, then the selected directory is ignored. The UI should probably reflect this somehow, maybe by greying out Directory:.
• Scope: in the deadgrep UI this should probably be moved near Directory:, probably just above it.

[maw]

Another annoyance: if you're using the in directory open buffers scope it shows full paths.