[Wilds] Continue toward a generalized "arc alias" workflow

Summary:
Ref T13098. This leaves a lot of rough edges but nothing is overtly broken so here's where we're at so far:

Config sources get "scopes", like user configuration vs system configuration. The major reason for this is so that arc set-config x y can know where it's supposed to write. This is generalized enough that we can implement arc set-config --system ... and arc alias --local ... and so on relatively easily later, although scopes themselves are not modular (a third-party can't add a new type of config scope). Maybe we'll modularize this some day but it felt like that's probably YAGNI/overboard since we have no current use cases. For now, a source does not need to belong to any particular scope.

Config may be writable (like user config in ~/.arcrc) or nonwritable (like --config flags). Writable config can now specify how to write to disk. Config files can actually write to disk now, although the only pathway for doing this that exists is via arc alias.

Aliases now parse properly and can write to disk. arc alias now lets you define aliases, and writes them to disk. The first time you do this, your ~/.arcrc file will be rewritten into a format which old arc can not read! It's relatively easily to unmangle/repair these files so I'm planning to just let this happen.

When a toolset is invoked, it now reads and evaluates aliases. Aliases have a lot of new guard rails like suggesting the user try arc draft if they type phage draft, allowing alias chains, detecting cycles, and limiting chain length.

Workflows can provide help and argument lists in a more structured way. I've moved this to sub-objects: help is now on WorkflowInformation (instead of a bunch of different getHelp(), getSynopsis() methods) and arguments now have a WorkflowArgument object instead of a dictionary. I think this pattern is generally better for extending: it lets us add and change stuff with less impact (and greater explicitness) down the road.

arc alias now has reasonable help text and argument documentation. The arc alias (list) and arc alias x (details/remove) flows don't work yet but arc alias x y does.

arc liberate now uses the new help/argument stuff, although the help needs more beef eventually. I pruned a bunch of long-obsolete or questionable flags and renamed --all to --clean since --all sounds like "liberate all libraries", which is now the default behavior of arc liberate.

Test Plan:
You can now define chains of aliases. Finally!

$ arc draft4
 WARNING  Ignoring unrecognized configuration option ("hosts") from source: User Config File (/Users/epriestley/.arcrc).
 WARNING  Ignoring unrecognized configuration option ("load") from source: Project Config File (/Users/epriestley/dev/core/.arcconfig).
 ALIAS  arc draft4 -> arc draft3
 ALIAS  arc draft3 -> arc draft2
 ALIAS  arc draft2 -> arc diff
Usage Exception: Unrecognized argument '--draft'.

This also works now:

$ phage alias deploy-secure -- deploy --hosts secure001-4 --limit 1

General!

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13098

Differential Revision: https://secure.phabricator.com/D19697