Have -pl only complete maven projects

This makes the completions for `-pl` cleverer, so it will only give completions for directories which are actually Maven modules (as indicated by the presence of a `pom.xml`).

It will also provide completions for modules which are several levels deep in the hierarchy, which has the nice effect of not considering `parent-module,` to be a valid completion when there's a module `parent-module/child-module` (you'll still get offered `parent-module`, i.e. without the trailing comma in this case, which makes it easier to type `/` to continue getting completions for submodules).

Informal timing numbers for `__find_mvn_projects` on some Atlassian code bases:

* JIRA (98 modules / 84477 files): ~0.150s
* Confluence (143 modules / 53855 files): ~0.100s
* Crowd (75 modules / 19219 files): ~0.050s

These numbers seem reasonable to me; you do notice a bit of a delay, but it's not a huge deal. This could be improved slightly when completing child module names by making `__find_mvn_projects` take `cur` into account so `find` starts its search in the already-typed-out parent module, but I figure that's something for a later pull request.

I did try an alternate approach using bash's [globstar](http://www.gnu.org/software/bash/manual/html_node/The-Shopt-Builtin.html) but performance was ~2-3x worse than using find, plus `globstar` was only added in bash 4 so `find` seems more portable to me.