Provide API for hacking around path problems with linking file descriptors

[jhump]

Due to golang/protobuf#567, file descriptor registration in the protobuf runtime is error-prone. If a file uses a relative path in an import statement that does not precisely match the path given to protoc to generate that file's corresponding Go code, then linking of descriptors can fail.

A possible work-around is to provide an API for users to register aliases for file paths. For example, a file may have been compiled with protoc and thus registered at runtime as foo.proto, but another file might import the same file via import "misc/foo.proto"; (e.g. with misc/ added to path). The API would allow users to register "misc/foo.proto" as an alias for "foo.proto". Since registration is global, the aliases can probably safely be global, too (e.g. registered in a package variable map [protected by mutex for thread-safety] that is queried from any calls to desc.LoadFileDescriptor).