Add messages for devtools functions when internet is disabled

[rosemm]

In addition to base::install.packages() there are a few popular functions in the devtools package that require internet access. In particular:

• install_github()
• source_url()

There are a number of other, related functions as well, though (for install_* functions especially).
In general, we may need a fair amount of ongoing support for this package as Arcus customers run into new functions that require internet (for some discussion: https://support.bioconductor.org/p/9154886/).