[Rd] Suggestion/opinions: add a `absolute` param to `normalizePath()` to force returning an absolute path

[Dean Attali]

This request stems off a bug report I posted
https://bugs.r-project.org/bugzilla/show_bug.cgi?id=17757 where it was
determined the current behaviour is as expected.

To recap: when given a real file, normalizePath() always* returns the full
absolute path. When given a non-existent file, normalizePath() returns a
full path on Windows but it returns the input on other systems*. I'd argue
that there are benefits to being able to reliably and consistently get a
full path, regardless of whether the file exists or not. In order to not
break existing behaviour, I propose adding an argument `absolute = FALSE`
that will attempt to return an absolute path when the argument is set to
TRUE. I don't have any evidence for this claim, but I believe that others
who use this function would expect, like I did, that an absolute path is
returned regardless of the file state. I understand the documentation is
correct because it warns the absolute path may not be returned, but I
believe it would be a useful feature to support.


* I've tested this on Win7, Win10, two versions of MacOS, ubuntu. This
behaviour may not be true in other OSes

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