Refactor ZIP importing

[HebaruSan]

Problems

During and after #4152, I noticed some issues with ZIP importing in GUI:

• It appears to freeze for quite a while before it shows a popup
• It's slow
• The progress bar for importing treats each file as the same fraction of total completion, even if one is 1 GiB and another is 1 KiB, and the updates can be infrequent

Since importing ZIPs is our go-to workaround when users report slow mod download speeds, it should be usable and performant.

Causes

• GUI.Main.ImportModules calls ModuleInstaller.ImportFiles on a foreground thread, so all UI updates pause until it finishes
• For mods that share a ZIP, the import stores the ZIP once for every such mod, which can involve deleting and re-writing the same file in the cache multiple times
• The import will copy each file into the cache and then ask if it can delete them, whereas if you're OK with losing the original, moving would be faster
• The files' hashes are checked twice, once to find the module associated with each file, and then again to validate them before storing in the cache, which is redundant

Changes

• Now GUI.Main.ImportModules uses Wait.StartWaiting to do the imports in a background thread, so the UI will be responsive
• Now the ZIPs are stored to cache once per URL, so mods that share URLs will no longer clobber one another
• Now the not-indexed files are reported together in one message
• Now the already-cached mods are reported together in one message
• Now if the user says it's OK to delete, the files are moved instead of copied into the cache
• Now the hashes are not re-validated, so storing is faster
• Now import progress bar updates are proportional to file size, so they feel nice and smooth and linear

This should make importing ZIPs in GUI more pleasant overall.