Add XDG Base Directory Specification support

[lordzsolt]

Motivation:

As discussed in #6204, users would like to have a clean home directory.
And in the discussions following the merge of #3430, it seems there is a willingness to adhere to the XDG spec, and support ~/.config/swiftpm.

Modifications:

If the XDG_CONFIG_HOME environmental variable is defined, use $XDG_CONFIG_HOME/swiftpm as the root dotSwiftPM directory.

Result:

The symlinks that were previously stored in ~/.swiftpm are now stored in $XDG_CONFIG_HOME/swiftpm when this variable is defined, therefore not cluttering the home directory of users.

Closes #6204

[MaxDesiatov]

We either need to use EnvironmentVariables.process() instead of ProcessInfo.processInfo.environment for improved Windows handling in the future, or exclude this logic on Windows altogether.

[MaxDesiatov]

@swift-ci test

[MaxDesiatov]

Thanks!

[MaxDesiatov]

@swift-ci test

[MaxDesiatov]

@swift-ci test windows

[alichtman]

Thank you!!

[bartekpacia]

Thank you for working on this!

Will this be released in Swift 6.0, or maybe earlier?

[MaxDesiatov]

5.10 has been released already. Swift 6.0 is branching off main tomorrow.