macOS: use ~/.config/pip/pip.conf as default as well (#4100)

[atdaemon]

This commit will use ~/.config/pip as config directory if ~/Library/Application Support/pip directory does not exist on macOS.
Updated doc as well.

Tested by adding a logger that lists all config files in get_config_files method -

$ ls ~/Library/Application\ Support/pip
ls: /Users/anish/Library/Application Support/pip: No such file or directory
$ pip 1>/dev/null
WARNING:pip.baseparser:get_config_files - ['/Library/Application Support/pip/pip.conf', '/Users/anish/.pip/pip.conf', '/Users/anish/.config/pip/pip.conf']
$ mkdir ~/Library/Application\ Support/pip
$ pip 1>/dev/null
WARNING:pip.baseparser:get_config_files - ['/Library/Application Support/pip/pip.conf', '/Users/anish/.pip/pip.conf', '/Users/anish/Library/Application Support/pip/pip.conf']
$ rm -rf ~/Library/Application\ Support/pip/
$ pip 1>/dev/null
WARNING:pip.baseparser:get_config_files - ['/Library/Application Support/pip/pip.conf', '/Users/anish/.pip/pip.conf', '/Users/anish/.config/pip/pip.conf']

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This change is 

[atdaemon]

Hi @xavfernandez , please take a look.
This one is for issue #4100
Thanks!

[xavfernandez]

Hello, as I have no knownledge of the macOS ecosystem I've no idea wheither this new location would make sense.
Maybe @dstufft or @pfmoore have an opinion on the topic ?

[pfmoore]

Sorry, I also have no knowledge of OSX

[tweekmonster]

@mhinz and myself have lots of knowledge of macOS. Yes, this location makes sense.

$HOME/Library/Application Support is typically for GUI applications and it's supposed to be the application's support files. Typically files generated by the application for the user's benefit. It's the equivalent of $XDG_DATA_HOME. This is probably the worst place for a command line tool's configs.

$HOME/Library/Preferences is the closest thing to $XDG_CONFIG_HOME. It's expected that the configs are named using a bundle identifier (e.g. io.pypa.pip) and that it's in the plist format. This is the default location when using the defaults command line tool, or NSUserDefaults in Cocoa applications: https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/UserDefaults/AboutPreferenceDomains/AboutPreferenceDomains.html

It's pretty annoying when third party command line tools use either of those directories. The file/directory names are often not what you expect making it hard to find or even discern if configs exist. They're also not tab completion friendly because these directories exist:

$HOME/Library/Application Scripts
$HOME/Library/Application Support
$HOME/PreferencePanes
$HOME/Preferences

It's easier on developers when config locations are predictable on all *nix OSes, macOS being no exception.

[xavfernandez]

After checking with a few mac users, they agreed it made more sense. Thanks.