Add directory argument

[interstateone]

This adds a directory argument at the top level (so it affects all commands) which determines the directory that xcodes installs in to. It defaults to /Applications, which is the current behaviour. xcodes will also check for a value in the XCODES_DIRECTORY environment variable. It applies to installation but also the other commands, so that e.g. selecting or uninstalling a version looks in the desired directory.

The motivation for all of this is to support users that may not want to store Xcode versions in /Applications, for example if they don't have enough space on their boot volume.

This also aligns with the addition of the directory argument as part of #121.

Testing

First, clone this repo if necessary and check out the PR branch:

git clone https://github.com/RobotsAndPencils/xcodes.git
cd xcodes
git checkout -b interstateone-xcodes-destination master
git pull https://github.com/interstateone/xcodes.git xcodes-destination

then run commands with either the directory argument or XCODES_DIRECTORY environment variable set. If it's set to a directory that doesn't exist, or to a path that isn't a directory, it should print an error.

Expected results:

❯ swift run xcodes list --directory /Users/brandon/Xcodes
Directory argument must be a directory, but was provided /Users/brandon/Xcodes.

# make sure the directory exists
❯ mkdir /Users/brandon/Xcodes

# it doesn't have any Xcodes in it yet
❯ swift run xcodes installed --directory /Users/Brandon/Xcodes

# download command still works like before
❯ swift run xcodes download 12.3 --directory /Users/brandon/Xcodes
(1/1) Downloading Xcode 12.3.0: 99%

Xcode 12.3.0 has been downloaded to /Users/brandon/Xcodes/Xcode-12.3.0.xip

❯ ls /Users/brandon/Xcodes | grep Xcode                               
-rw-r--r--   1 brandon  staff    11G  2 Jan 11:19 Xcode-12.3.0.xip

# install works
❯ XCODES_DIRECTORY=/Users/brandon/Xcodes swift run xcodes install 12.2                                  
(1/6) Downloading Xcode 12.2.0: 99%
(2/6) Unarchiving Xcode (This can take a while)
(3/6) Moving Xcode to /Users/brandon/Xcodes/Xcode-12.2.0.app
(4/6) Moving Xcode archive Xcode-12.2.0.xip to the Trash
(5/6) Checking security assessment and code signing
(6/6) Finishing installation
xcodes requires superuser privileges in order to finish installation.
macOS User Password: 

Xcode 12.2.0 has been installed to /Users/brandon/Xcodes/Xcode-12.2.0.app

❯ ls /Users/brandon/Xcodes | grep Xcode
drwxr-xr-x   3 brandon  staff    96B 23 Oct 20:24 Xcode-12.2.0.app
-rw-r--r--   1 brandon  staff    11G  2 Jan 11:19 Xcode-12.3.0.xip

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Closes #100

[csykes]

Verified locally using ENV variable + command line option