pkg_ref could take in the output of R CMD build

[llrs-roche]

R packages to be assessed might come on different forms. riskmetric already accounts for multiple ways: from uncompressed source folders, from CRAN, from Bioconductor and from libraries. However, one could have a compressed packages from R CMD build to assess them:

# Here I just download it to make a minimal reprex
download.file("https://cran.r-project.org/src/contrib/flow_0.2.0.tar.gz", destfile = "flow_0.2.0.tar.gz")
## trying URL 'https://cran.r-project.org/src/contrib/flow_0.2.0.tar.gz'
## Content type 'application/x-gzip' length 3790402 bytes (3.6 MB)
## downloaded 3.6 MB
## 
pkg_ref("flow_0.2.0.tar.gz")
## Error in determine_pkg_source(x, source, repos) : 
##   can't interpret character 'flow_0.2.0.tar.gz' as a package reference
```

I am asking for this because when trying to build reports for packages with the output of R CMD check and the assessment we might not want to have them (yet) on a library or uncompressed. The solution might be as easy as using `untar`, but I think it might be a nice to have feature given that riskmetric already provides several methods to ingest packages. 

Note: that I think `R CMD build` can be can be compressed with different methods, but I would start by accepting .tar.gz files, as this is the default and more common.