@davydden @tgamblin
This Issue is related to: #1924 #1940 #1561

Currently, the @develop version is special, in that it is larger than all numeric versions, whereas all other non-numeric versions are smaller. Although this does serve a purpose, it feels like a special-case "hack" where a more general (and transparent) approach could serve us better. This is a proposal for comparing versions that is more general.

A version consists of a series of strings separated by dots. For example: '1', '1.2', 'master', '1.2.develop', etc. Versions are compared by:

1. Split on the dots. Eg: '1.2.develop' --> ('1', '2', 'develop')
2. Compare the tuples element-by-element using the following rules:
   a) An element is numeric if it consists of all 0-9 digits (plus other stuff, like 2a).
   b) Any element starting '<' is always less than 0; any element starting with '>' is greater than 9.
   c) All other elements will be sorted in alphabetical order, and placed less than elements starting with '<'

In practice, two things would need to change from current use:

1. Replace '@develop' with '@>develop'
2. Use quotes when specifying these versions in the bash shell. (Unless we think of different symbols to use for < and >. The symbols can be more than one character if we like. Maybe '~' and '+' since those are already used elsewhere in Spack).

Thoughts?

[This proposal has been updated slightly from original]