Be more receptive to offline environments

[Kangie]

Current behaviour is to update the cached repositories.xml whenever an operation is performed, regardless of whether we need that information to continue (e.g. creating a new local repo, adding a repo by uri, removing an existing repo).

This commit changes this behaviour to only update the cache when performing operations that use the cache and adds a new OFFLINE_MODE configuration variable that skips fetching repositories.xml entirely.

Closes #19

[Kangie]

the TL;DR here is that now only enable and list try to update the repositories.xml by default (this is in line with existing documentation), and that can be overridden by OFFLINE_MODE.

Rationale:

• Disabling means you know the cached number or are using name
• Removing is the same
• Creation is an offline action
• Add provides name, sync-type, and uri

This came out of my recent experiences trying to use something as basic as eselect repository create in a firewalled environment; it's a really convenient helper that shouldn't rely on a webrequest succeeding to perform most functions.

[Kangie]

It's not quite there yet, as there's an edge case around 'what do we do if there isn't a cached xml', but I figured it was worth raising for comment while I iron the rough edges out.

Edit: What we do is create an empty XML file if we don't have a cached version to fall back on in offline mode, with predictable results - list only shows local repositories (though doesn't get remote URIs from repos.conf, hm, potential enhancement), and enable does nothing useful because we only know about local repositories.

[thesamesam]

(wrong button, meant to run CI)

[maffblaster]

What are we waiting on here? Code revisions from @Kangie?

[mgorny]

Yes. The comments weren't addressed.