more complete output format dependency detection

We introduced detection of output format packages here:

https://github.com/rstudio/packrat/pull/280/files

A couple more things need to be done to make this airtight:

1. All output formats that the Rmd could be rendered to need to be inspected (rather than just the default one). This will be possible in a future version of rmarkdown via the new all_output_formats function: https://github.com/rstudio/rmarkdown/commit/4f36bc0bd2b59343b7c80c0a79103b92b2ca97b7
2. There is technically a chance that a custom site generator defined in another package could be referenced in the "site" metadata field (it typically just has "site: true" and uses the default generator or one defined by it's output format however it's possible to pass an arbitrary function here).

We expect that the cases where #1 and #2 are required to correctly detect the dependencies will be extremely rare or non-existent so this isn't urgent.

Start with the files changed in pull request #280 and the referenced rmarkdown all_output_formats function. Trace how Rmd output formats and the site metadata field are currently inspected. Done means dependency detection covers every available output format and the possible custom site generator case.