* Add devel and fix head install for Elasticsearch
This commit adds a devel install for Elasticsearch, and fixes the head
install for Elasticsearch to reflect changes that have occurred in
master (e.g., changing the name of the plugin script to
"elasticsearch-plugin").
* Use devel? and head? in caveats and tests
This commit simplifies caveats and tests to use devel? and head? instead
of relying on external state to determine which command-line programs
and arguments to use.
Improves the caveats, dropping the assumption everyone installing Perl is
familiar enough with it to not be scared off by the somewhat brutal configuration
output.
This is an officially-supported mechanism to rewrite the hashbangs of scripts
generated with Perl. The upside here is that scripts generated with our Perl
won't break every single time the version is bumped or we revision the formula,
regardless of whether the update/revision actually causes incompatibility or not.
A lot of Perl users configure things locally, and we nudge them to do so in
the caveats, but our defaults should be a little more sane so users can use
Perl "out of the box" with minimal setup.
At the moment Perl only checks inside the Cellar for other Perl modules:
```
@INC:
/usr/local/Cellar/perl/5.24.0/lib/perl5/site_perl/5.24.0/darwin-thread-multi-2level
/usr/local/Cellar/perl/5.24.0/lib/perl5/site_perl/5.24.0
/usr/local/Cellar/perl/5.24.0/lib/perl5/5.24.0/darwin-thread-multi-2level
/usr/local/Cellar/perl/5.24.0/lib/perl5/5.24.0
```
To reduce the amount users need to carry custom variables around in their shell
profiles, let's tell Perl to check our prefix by default as well. This removes
the need to alter the environment manually when installing things like
Imagemagick's Perl element against Homebrew's Perl.
Ideally we would overwrite the sitelib, but Perl expects to be able to mkdir & copy
files into that prefix at build and our sandbox breaks that. It'd need a monster
hack to prevent that and handle it post_install, sadly.
Perl's configure script, attempting to be helpful, uses shorter installation
prefixes if it detects the top-level prefix argument contains the word "Perl"
in it.
Sadly, this doesn't really work for Homebrew because whilst the installation
is actually within the Cellar containing the word "Perl" everything is symlinked
into `/usr/local` (Presuming that's where your Homebrew is), which means instead
of Perl being available in i.e. `/usr/local/lib/perl5/site_perl/5.24.0` the libraries
get symlinked into `/usr/local/lib/5.24.0` which is completely opaque to end-users
and opens up the risk of conflict with anything else which behaves in the same
way.
This tweak shoves it back into the expected sort of installation structure.
