[build] Populate //zircon/public/banjo with fixed BUILD.gn files

Migrate to a new way to define the //zircon/public GN targets, starting
with just //zircon/public/banjo.  Instead of generating individual
//zircon/public/*/BUILD.gn files from templates, hard link all those
files to the same generic template.gn file.  That file uses its own
directory name as reported by GN's get_path_info() to look up its entry
in a JSON list written as a build API module by Zircon `gn gen`.

Bug: BLD-437 #comment Populate //zircon/public/banjo with fixed BUILD.gn files
Change-Id: I927009c031a8190d4b73ff9182e871bd4304fe39