Source tarballs not found with 'local.repos'

I have downloaded source tarballs for all dependencies of my project:

> head(dir('localsrc', recursive = TRUE))
[1] "AMGUtils/AMGUtils_0.69.tar.gz"      "assertthat/assertthat_0.2.0.tar.gz" "backports/backports_1.1.2.tar.gz"  
[4] "base64enc/base64enc_0.1-3.tar.gz"   "BH/BH_1.66.0-1.tar.gz"              "bindr/bindr_0.1.1.tar.gz"          

(this is the same kind of structure as packrat's own packrat/src/ directory.)

When I try to use this as a local.repos argument, the command fails:

> packrat::init(options=list(local.repos='localsrc'))
Initializing packrat project in directory:
- "~/git/loadfx"
Error in read.dcf(...) : cannot open the connection
In addition: Warning message:
In read.dcf(...) :
  cannot open compressed file 'localsrc/AMGUtils/DESCRIPTION', probable reason 'No such file or directory'

Is this not how the feature is intended to be used? If not - would it be easy to add? I'm wondering if the "directories containing package sources within sub-directories" mentioned in the local.repos docs are actually untarred source tarballs, which would seem a lot harder to manage (especially since it wouldn't let me pick from multiple versions of a package).

Reproduce the failure with packrat::init(options=list(local.repos='localsrc')) using the tarball layout shown in the issue. Trace the local.repos handling and compare it with the documented directory structure; done means the intended source-tarball behavior is supported or the documentation clearly matches the accepted layout.