add option to respect platform's cache directories

[jesteria]

…Rather than use a static path like ~/.cache/ (via --root) or ~/.shiv (the default).

The added shiv build option --platform-root will enable runtime determination of the platform- and executable-appropriate cache directory:

1. If the archive is understood to be globally-installed on the platform, and a known system-wide cache (such as /var/cache/) is already populated or writable by the current user, then this will be used.

2. If the platform is understood to feature a user-dedicated cache (such as ~/.cache/) – or specifies one via environment variable (such as XDG_CACHE_HOME) – then this will be used.

3. Otherwise, ~/.shiv will be used.

And, in cases (1) and (2), the cache root directory will be named according to the name of the archive – not referencing the toolset which created it (namely shiv) – for example...

Linux:

• /var/cache/cowsay/cowsay_3.03/site-packages/
• ~/.cache/cowsay/cowsay_3.03/site-packages/

Darwin / OS X:

• /Library/Caches/cowsay/cowsay_3.03/site-packages/
• ~/Library/Caches/cowsay/cowsay_3.03/site-packages/

Windows:

• ~/AppData/Local/cowsay/cowsay_3.03/site-packages/

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I'll note that I've followed the shiv issues on this and understand that this might not be of interest to pull – particularly now that an alternative root can be specified at build time via --root.

However, it's my impression that this functionality enables a shiv-built executable to do a significantly better job meeting the installation platform's expectations, at least on the systems I'm targeting, (and as a bonus without additional build-time configurations). As such, this is the functionality I'd prefer, by a long shot; and, I'd guess there's a not-insignificant number out there who would appreciate this, as well. Adding this as an "opt-in" feature – only enabled by build flag – seems appropriate, to me.

Cheers.

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See also: #141, #142.

[jesteria]

Update: In appreciation of #242 this has been expanded to a --platform-root option, which attempts to respect the expectations of OS X and Windows, (rather than merely those of Linux or XDG).

Importantly, I think, this changeset additionally:

• considers whether the executable has been globally installed and a system-wide cache is available (e.g. installed as /usr/bin/cowsay and /var/cache/cowsay/ is populated or writable)
• when writing to a platform-customary location, uses the name of the executable archive rather than the name "shiv" (e.g. /var/cache/cowsay/ or ~/.cache/cowsay/)