Create, install and relocate tarballs of installed packages

Adds the "buildcache" command to spack. The buildcache command is
used to create gpg signatures for archives of installed spack
packages; the signatures and archives are placed together in a
directory that can be added to a spack mirror. A user can retrieve
the archives from a mirror and verify their integrity using the
buildcache command. It is often the case that the user's Spack
instance is located in a different path compared to the Spack
instance used to generate the package archive and signature, so
this includes logic to relocate the RPATHs generated by Spack.

lib/spack/docs/binary_caches.rst

@@ -0,0 +1,108 @@
+.. _binary_caches:
+
+Build caches
+============
+
+Some sites may encourage users to set up their own test environments
+before carrying out central installations, or some users prefer to set
+up these environments on their own motivation. To reduce the load of
+recompiling otherwise identical package specs in different installations,
+installed packages can be put into build cache tarballs, uploaded to 
+your spack mirror and then downloaded and installed by others.
+
+
+Creating build cache files
+--------------------------
+
+A compressed tarball of an installed package is created. Tarballs are created
+for all of its link and run dependency packages as well. Compressed tarballs are
+signed with gpg and signature and tarball and put in a ".spack" file. Optionally
+, the rpaths ( and ids and deps on macOS ) can be changed to paths relative to 
+the spack install tree before the tarball is created.
+
+Build caches are created via:
+
+.. code-block:: sh
+
+   $ spack buildcache create 
+
+
+Finding or installing build cache files
+---------------------------------------
+
+To find build caches or install build caches, a spack mirror must be configured
+with
+ 
+``spack mirror add <name> <url>``. 
+
+Build caches are found via: 
+
+.. code-block:: sh
+
+   $ spack buildcache list
+
+Build caches are installed via:
+
+.. code-block:: sh
+
+   $ spack buildcache install 
+   
+
+Relocation
+----------
+
+Initial build and later installation do not necessarily happen at the same 
+location. Spack provides a relocation capability and corrects for RPATHs and 
+non-relocatable scripts. However, many packages compile paths into binary 
+artificats directly. In such cases, the build instructions of this package would
+need to be adjusted for better re-locatability.
+
+
+Usage 
+-----
+spack buildcache create <>
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+Create tarball of installed spack package and all dependencies. 
+Tarballs is checksummed and signed if gpg2 is available.
+Places them in a directory build_cache that can be copied to a mirror.
+Commands like "spack buildcache install" will search it for pre-compiled packages.
+ 
+
+options:
+
+-d <path> : directory in which "build_cache" direcory is created, defaults to "."
+-f : overwrite ".spack" file in "build_cache" directory if it exists
+-k <key> : the key to sign package with. In the case where multiple keys exist, the package will be unsigned unless -k is used.
+-r : make paths in binaries relative before creating tarball
+-y : answer yes to all create unsigned "build_cache" questions
+<> : list of package specs or package hashes with leading /
+
+spack buildcache list <>
+^^^^^^^^^^^^^^^^^^^^^^^^
+Retrieves all specs for build caches available on a spack mirror.
+
+options:
+
+<> string to be matched to matched to begining of listed concretized short 
+specs, eg. "spack buildcache list gcc" with print only commands to install gcc
+package(s)
+
+spack buildcache install <>
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+Retrieves all specs for build caches available on a spack mirror and installs build caches 
+with specs matching the specs or hashes input. 
+
+options:
+
+-f : remove install directory if it exists before unpacking tarball
+-y : answer yes to all to don't verify package with gpg questions
+<> : list of package specs or package hashes with leading /
+
+spack buildcache keys
+^^^^^^^^^^^^^^^^^^^^^
+List public keys available on spack mirror.
+
+options:
+
+-i : trust the keys downloaded with prompt for each
+-y : answer yes to all trust all keys downloaded

lib/spack/docs/index.rst

@@ -65,6 +65,7 @@ or refer to the full manual below.
    repositories
    command_index
    package_list
+   binary_caches
 
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    :maxdepth: 2