Method for identifying packages by name

[wch]

It would be nice to make something like this work:

reload('ggplot2')
# Error: Can't find package ggplot2

And also have a more informative error message.

[hadley]

If we go down this route, we need some way of indicating which devtools functions work with packages and which with paths.

[wch]

Implemented in #126, with inst().