(re-)allow include_dependency(directory)

[benlorenz]

This would allow using a directory for include_dependency(path) to track whether any file or subdirectory is added.

This was working fine (for Oscar.jl) in Julia 1.6 and 1.8 but since #47184 (in master and 1.9) the cache file is rejected with the following error and a fresh precompilation is triggered every time Oscar is loaded:

┌ Debug: Rejecting stale cache file <...>/aJ3Pg_wOMvI.ji because file <...>/Oscar.jl/experimental does not exist

Here experimental is a subfolder where developers can add extra code that will then be picked up without adding all files explicitly to some list in the main Oscar sources.

I am aware that the docs for include_dependency only mention file, I understood file here in a more file-system oriented way and not strictly as regular file, similar to how it is used in the docs for stat.

I adjusted the docs for include_dependency accordingly and clarified that this uses the mtime of path.

cc: @lkastner
x-ref: oscar-system/Oscar.jl#2203

[vtjnash]

I think this is okay to change back, since it should adequately reflect any changes to readdir. The JLL packages should be doing this too, for correct execution. If we ever add some content-based hashing as well, we can state that the content is the set of filenames returned by readdir and that should be consistent with this mtime.

[benlorenz]

The test-failure on win32 looks unrelated, worker 5 died with std::bad_alloc during Tar.jl tests.