[ISSUE] ejabberd overwrites system Elixir

[0n-s <0n-s@users.noreply.github.com>]

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New issue by 0n-s on void-packages repository

https://github.com/void-linux/void-packages/issues/21494

Description:
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### System

* xuname:  
  Void 5.4.35_1 x86_64 AuthenticAMD uptodate rrrmFFFFF
* package:  
  ejabberd-20.02_2

### Expected behavior
ejabberd should use the elixir package in XBPS instead of installing its own. If it requires a specific version, it could be packaged & work with xbps-alternatives to allow usage of the latest elixir, or at least not overwrite each other's files & not break each other (like how the guile & guile1.8 packages work).
### Actual behavior
ejabberd installs its own Elixir distribution (which, by the way, is a rather old 1.4.4, in comparison to the elixir package's 1.10.2), as well as even overwriting some of the elixir package's binaries. I doubt this is an Elixir 1.4.4 problem, but it causes `elixir(1)` to encounter some concerning-looking problems even with a simple `elixir -v`:
```
$ elixir -v
=SUPERVISOR REPORT==== 30-Apr-2020::20:05:34.064575 ===
    supervisor: {local,'Elixir.Logger.Supervisor'}
    errorContext: start_error
    reason: noproc
    offender: [{pid,undefined},
               {id,'Elixir.Logger.ErrorHandler'},
               {mfargs,
                   {'Elixir.Logger.Watcher',watcher,
                       [error_logger,'Elixir.Logger.ErrorHandler',
                        {true,false,500}]}},
               {restart_type,permanent},
               {shutdown,5000},
               {child_type,worker}]
=CRASH REPORT==== 30-Apr-2020::20:05:34.089540 ===
  crasher:
    initial call: application_master:init/4
    pid: <0.85.0>
    registered_name: []
    exception exit: {{shutdown,
                         {failed_to_start_child,'Elixir.Logger.ErrorHandler',
                             noproc}},
                     {'Elixir.Logger.App',start,[normal,[]]}}
      in function  application_master:init/4 (application_master.erl, line 138)
    ancestors: [<0.84.0>]
    message_queue_len: 1
    messages: [{'EXIT',<0.86.0>,normal}]
    links: [<0.84.0>,<0.44.0>]
    dictionary: []
    trap_exit: true
    status: running
    heap_size: 376
    stack_size: 27
    reductions: 193
  neighbours:

=INFO REPORT==== 30-Apr-2020::20:05:34.102432 ===
    application: logger
    exited: {{shutdown,
                 {failed_to_start_child,'Elixir.Logger.ErrorHandler',noproc}},
             {'Elixir.Logger.App',start,[normal,[]]}}
    type: temporary
Erlang/OTP 22 [erts-10.7] [source] [64-bit] [smp:1:1] [ds:1:1:10] [async-threads:1] [hipe]

Elixir 1.4.4
```
Using `xbps-install -f elixir` & then executing the command once again is business as usual:
```
elixir -v
Erlang/OTP 22 [erts-10.7] [source] [64-bit] [smp:1:1] [ds:1:1:10] [async-threads:1] [hipe]

Elixir 1.10.2 (compiled with Erlang/OTP 22)
```
Notice, also, the modified binaries in ejabberd's install:
```
$ ls -l /usr/bin/elixir /usr/bin/mix /usr/bin/iex | cut -c 42-
/usr/bin/elixir
/usr/bin/iex
/usr/bin/mix
```
These were symlinks in the elixir package, not files!
```
$ xbps-query -Rf elixir | grep -F '/usr/bin/'
/usr/bin/elixir -> /usr/lib/elixir/bin/elixir
/usr/bin/elixirc -> /usr/lib/elixir/bin/elixirc
/usr/bin/iex -> /usr/lib/elixir/bin/iex
/usr/bin/mix -> /usr/lib/elixir/bin/mix
```
In addition, the extra Elixir install (which should really be in its own package, & that's only if ejabberd doesn't work with the current version; docs do not seem to mention incompatibility with latest versions, correct me if I'm wrong):
```
$ ls /usr/lib/elixir* -d
/usr/lib/elixir-1.4.4  /usr/lib/elixir
```
### Steps to reproduce the behavior
Install elixir, then ejabberd. The overwriting works in reverse as well, but not the other effects (e.g. `elixir -v` should work).