Added a sub-command to show if packages are relocatable (#9199)

* Added the `spack buildcache preview` sub-command

This is similar to `spack spec -I` but highlights which nodes in a DAG
are relocatable and which are not.

spec.tree has been generalized a little to accept a status function,
instead of always showing the install status

The current implementation works only for ELF, and needs to be
generalized to other platforms.

* Added a test to check if an executable is relocatable or not

This test requires a few commands to be present in the environment.
Currently it will run only under python 3.7 (which uses Xenial instead
of Trusty).

* Added tests for the 'buildcache preview' command.

* Fixed codebase after rebase

* Fixed the list of apt addons for Python 3.7 in travis.yaml

* Only check ELF executables and shared libraries. Skip checking virtual or external packages. (#229)

* Fixed flake8 issues

* Add handling for macOS mach binaries (#231)

.travis.yml

@@ -55,6 +55,24 @@ jobs:
       os: linux
       language: python
       env: TEST_SUITE=unit
+      addons:
+        apt:
+          packages:
+          - cmake
+          - gfortran
+          - graphviz
+          - gnupg2
+          - kcov
+          - mercurial
+          - ninja-build
+          - perl
+          - perl-base
+          - realpath
+          - patchelf
+          - r-base
+          - r-base-core
+          - r-base-dev
+
     - python: '3.6'
       sudo: required
       os: linux