Use softfail setting for importing remote packages.  This resolves issues such as

fatal error: Can't find overlay named NaOH.link in [:]: file atpkg/src/Package.swift, line 375

(when it is not required as it's a test-only dependency)

atpm needs to parse packages that refer to overlays in imported packages before those imported overlays are available

Don't treat those as hard failures

Swift preview 6

[atpm] `if-including` and `--include`

See https://github.com/AnarchyTools/atpm/issues/12

Parse the `if-including` directive in ExternalDepency
Reference a package in ExternalDependency, this is used for namespacing

[atpm] Allow SSH urls

SSH urls don't fit into the atfoundation scheme and so we have to use strings instead

atpkg support for binary atpm

See https://github.com/AnarchyTools/atpm/issues/1

This commit adds

* Dependency type to ExternalDependency, to distinguish between Git and Manifest types
  This design was settled on after looking at several, including autodetecting git (which is possible via git ls-remote)
  However, manifest ExternalDependency is quite different than Git ExternalDependency for several reasons (including the use of channels)
* Channels, which are essentially like micropackages inside a package.  A channel lets us say "stable" "beta" "linux" "osx" "swift2.2" etc.  All releases are part of a channel.  It would be possible to use separate manifests for each channel, but this would require the use of multiple manifests for basic osx/linux packages, which is undesireable
* Tokens can now start with numbers, which is important, as we now have syntax where `:1.0 { :url "https://example.com/some/tarball.xz"}` is used to define where some version can be located

Currently, allKeys can get out of sync with the actual task.  This can occur if you use the subscript setter.

This does not arise very often, but can occur if you have an attool (which has no fixed set of keys, so it uses allKeys) *and* you are setting keys from overlays or via some other method that modifies the keys for tasks after their creation.

Add only-platforms

This is a new task-level property to allow the skipping of tasks on platforms other than the ones specified

Swift optionals exist for a reason; to model the case where we don't have a value.

Atbuild now cares about not having a value here, therefore let's model it correctly.

Support escape code sequences. 

Note that this relies on https://github.com/AnarchyTools/atfoundation/pull/7,
which is not yet merged

Note that this relies on https://github.com/AnarchyTools/atfoundation/pull/7,
which is not yet merged