Fixed #224: added sites managment.

[Ninjapouet]

It adds a [sites] section in packages' description. In this section, one can declare the sites to use and what files must be installed in it. As we follow the general dune semantic, only the following root sites are available:

• lib
• bin
• sbin
• toplevel
• share
• etc
• stublibs
• doc
• man

Each of those fields takes an array of site specification with the following fields:

• root: a flag indicating if we use the root section or not (only for lib and share, default is false)
• exec: a flag indicating if files installed in this site must have the executable flag set
• dir: the directory name of the site
• install: the files to be installed in this site.

The installation specification is a table with the following fields:

• source: the source file, directory or glob to install
• destination: where to install source
• recursive: recursively install the subdirectories.

For example,

[[sites.share]]
dir = "www"
[[sites.share.install]]
source = "javascript/*.js"
destination = "js"

will install all javascript files in the javascript dir of the package into <prefix>/share/<package>/www/js directory.

The custom installed files are dynamically available at runtime through the module Sites. The module name can be changed
with the field name of section sites. The module itself exports an inner Sites module with values corresponding to the files
installed. With the previous example :

[sites]
name = "mysites"

Will provide the value Mysites.Sites.www which is a list of the installed javascript files.

[Ninjapouet]

Missing documentation at least, this is just for general approval.

[Ninjapouet]

This PR needs drom-share corresponding MR

[Ninjapouet]

Added documentation.

[Ninjapouet]

I'm not very satisfied with it because sites has meaning only for installing files so we need to handle both sites and installation rules.

[Ninjapouet]

Added the installation rules.

[Ninjapouet]

Changed version guard to 0.9.3, the future next version.