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

lib/spack/llnl/util/filesystem.py

@@ -47,6 +47,7 @@
     'install_tree',
     'is_exe',
     'join_path',
+    'last_modification_time_recursive',
     'mkdirp',
     'partition_path',
     'prefixes',
@@ -920,6 +921,15 @@ def set_executable(path):
     os.chmod(path, mode)
 
 
+def last_modification_time_recursive(path):
+    path = os.path.abspath(path)
+    times = [os.stat(path).st_mtime]
+    times.extend(os.stat(os.path.join(root, name)).st_mtime
+                 for root, dirs, files in os.walk(path)
+                 for name in dirs + files)
+    return max(times)
+
+
 def remove_empty_directories(root):
     """Ascend up from the leaves accessible from `root` and remove empty
     directories.