Provide basic target mapping system for packaging rules (pkg_filegroup)

[nacl]

Originally brought up in #36, each packaging rule currently has its own scheme for laying out package contents, each with its own advantages and disadvantages.

Practically speaking, these are all just variations on ensuring that bazel targets are consistently mapped into the output package. There should be a unified interface for this.

This is primarily the subject of the files in pkg/experimental, which represent a first pass at implementing this feature. We believe we have identified the various shortcomings of this scheme, and an new, more streamlined, one is currently in progress.

This is tracking various pull requests other issues related to this mapping system, namely:

• Backend providers (PackageFilesInfo, etc.) Provide providers for new packaging rule interfaces #246
• File mapper (pkg_files) Create rules for emitting PackageFilesInfo, PackageDirsInfo #273
• Directory mapper (pkg_mkdirs) Create rules for emitting PackageFilesInfo, PackageDirsInfo #273
• Symbolic link support (pkg_mklink) Provide pkg_mklink, pkg_filegroup #293
• Grouping support (pkg_filegroup) Provide pkg_mklink, pkg_filegroup #293
• Generic attribute support Allow for more structured package mapping rule attributes #290

This will cover most cases. Other issues that should to be addressed at some point include (we are encountering issues with this internally):

• strip_prefix on TreeArtifacts (this is a problem we're encountering internally) Packaging strip_prefix does not function on or within TreeArtifacts (directories) #269

Other, perhaps useful things that aren't as urgent:

• Default attribute derivation from inputs Try to derive default file mode from input types #302