Support GitHub sha as version

[hadley]

@kevinushey I think this is worth taking a look at now.

I'm not sure how to test it. Interactively, I did this in a new project:

pak::pak("rstudio/renv#1327")
renv::init()

unlink("renv/library", recursive = TRUE)
source("renv/activate.R")

[hadley]

We also need to think through if this is going to cause problems for us, given that I, at least, usually install renv with RStudio, not install_github() so it doesn't have an associated sha.

[kevinushey]

Overall LGTM

[kevinushey]

Maybe we should bring in R/version.R here:

renv/R/zzz.R

Lines 113 to 116 in 8b145e8

	# read the necessary bootstrap scripts
	scripts <- c("R/bootstrap.R", "R/json-read.R")
	contents <- map(scripts, readLines)
	bootstrap <- unlist(contents)

and use those utilities here? I'm not sure what we would call this function though; maybe it suffices to just check renv_version_length(version) == 3L to infer that it's a release version of renv?

[hadley]

It still needs slightly different code because it's slightly different test to figure out if we're using a sha or a version number. So IMO, it's not worth it.