Spack can automatically remove unused specs (#13534)

* Spack can uninstall unused specs

fixes #4382

Added an option to spack uninstall that removes all unused specs i.e.
build dependencies or transitive dependencies that are left
in the store after the specs that pulled them in have been removed.

* Moved the functionality to its own command

The command has been named 'spack autoremove' to follow the naming used
for the same functionality by other widely known package managers i.e.
yum and apt.

* Speed-up autoremoving specs by not locking and re-reading the scratch DB

* Make autoremove work directly on Spack's store

* Added unit tests for the new command

* Display a terser output to the user

* Renamed the "autoremove" command "gc"

Following discussion there's more consensus around
the latter name.

* Preserve root specs in env contexts

* Instead of preserving specs, restrict gc to the active environment

* Added docs

* Added a unit test for gc within an environment

* Updated copyright to 2020

* Updated documentation according to review

Rephrased a couple of sentences, added references to
`spack find` and dependency types.

* Updated function naming and docstrings

* Simplified computation of unused specs

Since the new approach uses private attributes of the DB
it has been coded as a method of that class rather than a
freestanding function.

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