[Modules] Improved module message report

[Xavier D. <xav...@gm...>]

Hello,

Module sub-commands like `load`, `unload` or `switch`, may perform
multiple load or unload modulefile evaluations in a row. Also these
kind of evaluation modes may sometimes trigger additional load or
unload evaluations, when for instance a modulefile contains a `module
load` command.

To improve the readability of the module messages produced relatively
to a load or an unload evaluation, these messages are now stacked
under a *Loading* or an *Unloading* message block that gathers all the
messages produced for a given modulefile evaluation:

  $ module load --no-auto foo
  Loading foo/1.2
    ERROR: foo/1.2 cannot be loaded due to missing prereq.
      HINT: the following module must be loaded first: bar/4.5

In addition, foreground `load`, `unload`, `switch` and `restore`
actions (ie. asked on the command-line) now report a summary of the
additional load and unload evaluations that were eventually triggered
in the process:

  $ module load --auto foo
  Loading foo/1.2
    Loading requirement: bar/4.5

This change is available in git repository and was the last addition
to the imminent v4.2 release.

Cheers,
Xavier