Add a new package Exa (#16126)

* Add new package exa

* Format and fix a silly typo

* Fix SHA256 SUM and make URL calculation dynamic

* Remove unnecessary URL calculation

* Update package.py

* Keep the version on 1 line

* Pass flake8 checks

var/spack/repos/builtin/packages/exa/package.py

@@ -0,0 +1,21 @@
+# Copyright 2013-2020 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+from spack import *
+
+
+class Exa(Package):
+    """exa is a replacement for ls written in Rust."""
+
+    homepage = 'https://the.exa.website'
+    url = 'https://github.com/ogham/exa/archive/v0.9.0.tar.gz'
+
+    version('0.9.0', sha256='96e743ffac0512a278de9ca3277183536ee8b691a46ff200ec27e28108fef783')
+
+    depends_on('rust')
+
+    def install(self, spec, prefix):
+        cargo = which('cargo')
+        cargo('install', '--root', prefix, '--path', '.')