Unfriendly error for '@include zoo' instead of '@import zoo'

[MichaelChirico]

The logic for processing @include could go for a sanity check. Using @include zoo by mistake gives some hard-to-trace error:

Warning in normalizePath(file.path(path, "R", collate)) :
  path[3]="...R/zoo": No such file or directory
Warning in file(con, "r") :
  cannot open file '...R/zoo': No such file or directory
Error in file(con, "r") : cannot open the connection
Calls: Main ... parse_package -> lapply -> FUN -> read_lines -> <Anonymous> -> file
In addition: Warning message:
'Collate' must contain a list of .R files

In retrospect this makes sense, but it took quite some time to debug. Is it valid to @include a non-R file? Couldn't we check file.exists() for @include directives to prevent this with a friendlier error?

[hadley]

Yup, could definitely do better here.

[hadley]

This is a little trickier than I had thought because @include is handled specially (since it needs to be run before loading the package). Will need some custom code in generate_collate() to detect files that aren't found in R/, warn, and drop from the collate.

[hadley]

Whoops, accidentally committed part of an experiment in #1293