More standardized location for "last-checked" file

[itamarst]

On the assumption that pip has done the relevant research correctly:

1. For Unix-y systems, use "$XDG_CACHE_HOME/app-name" or fallback to "~/.cache/app-name". (https://github.com/pypa/pip/blob/dec279ecdf8fd2b0f984dd213198635fdbe320c7/src/pip/_vendor/platformdirs/unix.py#L86)
2. For Windows, CSIDL_LOCAL_APPDATA (https://github.com/pypa/pip/blob/dec279ecdf8fd2b0f984dd213198635fdbe320c7/src/pip/_vendor/platformdirs/windows.py#L62)
3. For macOS, "~/Library/Caches/app-name" (https://github.com/pypa/pip/blob/dec279ecdf8fd2b0f984dd213198635fdbe320c7/src/pip/_vendor/platformdirs/macos.py#L37).

[itamarst]

https://docs.rs/app_dirs2/ or https://docs.rs/directories/ might have the right API to get this info.

[itamarst]

Other problems with current naming scheme:

1. A PyPI package might have same name as Crates package, but they will share same file on disk.
2. In theory files could conflict with files from other tools.
3. GitHub repos only use the name, not the owner, so namespace conflicts are possible (e.g. user1/fish and user2/fish would both be recorded as file with name fish).

My proposal, given cache dir obrained from app_dirs2 or the like: {cache_dir}/update-informer/{registry}-{owner}-{name}-{version} if owner is set, otherwise {cache_dir}/update-informer/{registry}-{name}-{version}.

[itamarst]

Further reading: it seems the standards on Windows at least involve application developer, which is ... not something the library should set, I guess? So maybe that should be an input.

[itamarst]

But... developer name can be optional with directories, at least.

[mgrachev]

My proposal, given cache dir obrained from app_dirs2 or the like: {cache_dir}/update-informer/{registry}-{owner}-{name}-{version} if owner is set, otherwise {cache_dir}/update-informer/{registry}-{name}-{version}.

Why did you decide to include the version to the file name?