[R-SIG-Mac] Package management: use zip, accept tar, accept package folders

[Timothy Bates]

The package installer seems somewhat restrictive at present.

I downloaded the "rgl.tar.tgz" file, and Safari automatically unpacked it to
tar (helpful hint: to stop this, option click the link, and it will simply
be downloaded "as is" to the download folder, without post processing).

R then barfed on the .tar package, saying it was not a tgz, and then finding
a (spurious?) error on the tar call.In fact is was a valid tar, which untars
just fine.

Suggestion: look at the extension on local file imports, and behave
appropriately: if it is "tar" then skip the unzip action.

Also, .zip (the OS X archive default) is not a legal compression: only the
close relative tgz is accepted: is this unnecessarily restrictive?

Finally, installing from a  local package directory seems broken: if you
select the package folder, it is simply ignored (error = no package
selected) 
If you select the package folder's container, you get the error "no packages
found". Both seem buggy to me?

Tim