Provide pkg_mklink, pkg_filegroup

[nacl]

This change provides the last two pieces of the basic package creation puzzle: a
symbolic link provisioning rule (pkg_mklink) and a simple package grouping
rule (pkg_filegroup).

As compared to the original rules in pkg/experimental/pkg_filegroup.bzl:

• pkg_mklink (formerly pkg_mklinks) now operates on a single
  source-destination pair. This is to ease integration of a planned feature
  where you will be able to make symbolic links to targets named by a label.
  The mechanics of this particular feature are not fleshed out at this time, but
  the current feeling is that these will be resolved as a part of the
  pkg_filegroup rule.

  To create multiple links, you can use a list comprehension in a BUILD
  file with minimal loss of clarity.

• pkg_filegroup is now an example of a "Grouping Rule", which allows for common
  groups of packaged files to be associated under a common name and prefix. These
  are intended to be inputs to packaging rules (pkg_rpm, pkg_tar, etc.).

Tests are included for all of the above.

[nacl]

Latest change modifies the PackageFilegroupInfo provider to contain a list of (child_provider, provider_origin) tuples, which will aid diagnostics in packaging rules without being too disruptive to other parts of the ruleset.

This seemed to be a reasonable compromise between intrusiveness and clarity, but I'm willing to entertain other options, like embedding the origins directly in the mapping providers (e.g. PackageFilesInfo).

[nacl]

@aiuto Ping. Is there anything else you'd like to see done as a part of this PR?

[aiuto]

It feels like the origin should be in the info, rather than as a tuple, but I'm willing to face that choice after we try to use it for some more complex things. I'm not worried about backwards compatibility of these providers while we are pre 1.0, because only you and I should be using them.