Automatically handle dynamic dependencies of static targets

[wlandau]

Prework

• Read and abide by drake's code of conduct.
• Search for duplicates among the existing issues, both open and closed.

Description

At #685, #1064, #1090, and #1103, I said that downstream targets of dynamic targets must be dynamic except if you use readd() to turn a dynamic target into a static one. This was always problematic because it required the user to understand the internals of dynamic branching. I just had an idea about how to avoid this problem. It has to do with the internals of memory management, and if it works, we will be able to write plans like this:

drake_plan(
   raw = mtcars[seq_len(4), ],
   rows = target(raw[, c("mpg", "cyl")], dynamic = map(raw)),
   means = colMeans(rows) # no readd()
)

Currently it fails because drake thinks rows is a vector of hashes (i.e. how it represents dynamic targets). The goal for this example is to get drake to understand that rows should be a data frame for means.

cc @dpmccabe, @keondonB, @billdenney, @brendanf

[brendanf]

Is there still an exported function to get the vector of keys to the subtargets? Here I'm thinking of my combine_dynamic_diskframe in #1076 (comment).

[wlandau]

Yes, the subtargets() function does this and is still available.