Set up environment before post-fetch actions (#456)

Set up environment before post-fetch actions (#456)

* Alire.Environment: take release paths from a Root

The path lookup has three levels:
1. For the root release, the own root folder
2. For a regular release, the dependencies folder "/" release unique folder
3. For a pinned softlink, the absolute path of the softlink

The root contains the solution which in turn contains the softlinks, so the
query for the path can be directly made to the root.

* Set up build env before deploying dependencies

* Tests for softlink nested path and env export

* Review fixes

* Alire.Environment.Load: Remove unused parameter

This flies by CE but is detected by FSF (?)

* Alire.Roots: move expression functions to body

There's some bug manifesting when a lot of code is in the spec in which the
linker complaints about missing references.

* Spelling fixes

* Updated new tests index versions

* Rename dir_separator/path_separator to match GNAT...

... in the Python testsuite

* Remove trailing '/' separator in extra prj paths

This is to match the output of full path normalization by Ada.Directories.