The builder configuration has proven useful for supporting generic
pre-entrypoint commands such as dependency managers not otherwise
supported by specific Blubber configuration. Adding additional
`builder.requirements` config expands support for such commands by
allowing the user to specify files that should be copied into the image
before the builder command runs.

To support this extra configuration, `builder` had to be changed from a
simple string to a mapping. The builder command must now by given as
`builder.command`.

The pattern of creating parent directories, copying files, and executing
one or more commands prior to the entrypoint has become a common
one. Some of the implementation of this pattern was moved from
`PythonConfig` into shared build macros `build.SortFilesByDir` and
`build.SyncFiles`. All config types that must have requirements files
copied over independently of the entire source tree (`PythonConfig`,
`BuilderConfig`, `NodeConfig`) now delegate to these functions.

Change-Id: I67f33034f22cee2851ec866cfb07ab20c23eba8c