Environments: specify packages for developer builds (#15256)

* allow environments to specify dev-build packages

* spack develop and spack undevelop commands

* never pull dev-build packges from bincache

* reinstall dev_specs when code has changed; reinstall dependents too

* preserve dev info paths and versions in concretization as special variant

* move install overwrite transaction into installer

* move dev-build argument handling to package.do_install

now that specs are dev-aware, package.do_install can add
necessary args (keep_stage=True, use_cache=False) to dev
builds. This simplifies driving logic in cmd and env._install

* allow 'any' as wildcard for variants

* spec: allow anonymous dependencies

raise an error when constraining by or normalizing an anonymous dep
refactor concretize_develop to remove dev_build variant
refactor tests to check for ^dev_path=any instead of +dev_build

* fix variant class hierarchy

var/spack/repos/builtin.mock/packages/dependent-of-dev-build/package.py

@@ -0,0 +1,17 @@
+# 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)
+
+
+class DependentOfDevBuild(Package):
+    homepage = "example.com"
+    url = "fake.com"
+
+    version('0.0.0', sha256='0123456789abcdefgh')
+
+    depends_on('dev-build-test-install')
+
+    def install(self, spec, prefix):
+        with open(prefix.filename, 'w') as f:
+            f.write("This file is installed")

var/spack/repos/builtin.mock/packages/dev-build-test-dependent/package.py

@@ -0,0 +1,29 @@
+# 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)
+
+
+class DevBuildTestDependent(Package):
+    homepage = "example.com"
+    url = "fake.com"
+
+    version('0.0.0', sha256='0123456789abcdefgh')
+
+    phases = ['edit', 'install']
+
+    filename = 'dev-build-test-file.txt'
+    original_string = "This file should be edited"
+    replacement_string = "This file has been edited"
+
+    depends_on('dev-build-test-install')
+
+    def edit(self, spec, prefix):
+        with open(self.filename, 'r+') as f:
+            assert f.read() == self.original_string
+            f.seek(0)
+            f.truncate()
+            f.write(self.replacement_string)
+
+    def install(self, spec, prefix):
+        install(self.filename, prefix)