Director based cache health tests

[haoming29]

Closes #545

This PR has two major changes. One is to add unauthenticated director-based health tests for caches. Second is to have a large overhaul of project structure to 1. minimize chances for cyclic import. 2. Establish a convention on internal dependencies and naming practice.

The first task happens over the most of the commits. The second task happens in 3c17dd8, dd13fc3, 8192935, 952229b, a8dce19, 4f444ba, where 4f444ba should contain most of the changes to the whole project while others are small changes to a specific directory.

The task two implemented proposed ideas in Pelican Folder Structure v7.7.0, including:

• Renamed cache_ui --> cache, origin_ui --> origin
• Renamed common --> server_structs
• Renamed director.go --> director_ui.go, redirect.go --> director in director package
• Renamed server_ui --> launcher_utils
• Removed cache dependency on director by moving NamespaceV1/V2 conversion functions from director to server_structs (old name common)
• Moved XRootDServer structs from server_utils to server_structs
• Moved origin export related code from common to server_utils

[jhiemstrawisc]

Interesting, do we have some underlying guarantee that this slice is ordered? Or how do we know we're leaving the "right" two files? Also, the logic here might be a more clear at first glance if you do if idx < len(dirItems)-2 to get rid of the extra -1.

[haoming29]

ReadDir sorts the dirEntry in order. The filenames are in format of cache-test-<date>.txt where date is a comparable string, so here I'm looking for the last two files, which have the largest date as its filename (or the latest files). I'll add a comment here to explain a bit

[jhiemstrawisc]

LGTM!