Use spack commands --format=bash to generate shell completion (#14393)

This PR adds a `--format=bash` option to `spack commands` to
auto-generate the Bash programmable tab completion script. It can be
extended to work for other shells.

Progress:

- [x] Fix bug in superclass initialization in `ArgparseWriter`
- [x] Refactor `ArgparseWriter` (see below)
- [x] Ensure that output of old `--format` options remains the same
- [x] Add `ArgparseCompletionWriter` and `BashCompletionWriter`
- [x] Add `--aliases` option to add command aliases
- [x] Standardize positional argument names
- [x] Tests for `spack commands --format=bash` coverage
- [x] Tests to make sure `spack-completion.bash` stays up-to-date
- [x] Tests for `spack-completion.bash` coverage
- [x] Speed up `spack-completion.bash` by caching subroutine calls

This PR also necessitates a significant refactoring of
`ArgparseWriter`. Previously, `ArgparseWriter` was mostly a single
`_write` method which handled everything from extracting the information
we care about from the parser to formatting the output. Now, `_write`
only handles recursion, while the information extraction is split into a
separate `parse` method, and the formatting is handled by `format`. This
allows subclasses to completely redefine how the format will appear
without overriding all of `_write`.

Co-Authored-by: Todd Gamblin <tgamblin@llnl.gov>

share/spack/qa/run-unit-tests

@@ -23,7 +23,7 @@
 ORIGINAL_PATH="$PATH"
 
 . "$(dirname $0)/setup.sh"
-check_dependencies ${coverage} git hg svn
+check_dependencies $coverage git hg svn
 
 # Move to root directory of Spack
 # Allows script to be run from anywhere
@@ -46,7 +46,7 @@ extra_args=""
 if [[ -n "$@" ]]; then
     extra_args="-k $@"
 fi
-${coverage_run} bin/spack test -x --verbose "$extra_args"
+$coverage_run bin/spack test -x --verbose "$extra_args"
 
 #-----------------------------------------------------------
 # Run tests for setup-env.sh
@@ -57,15 +57,18 @@ export PATH="$ORIGINAL_PATH"
 unset spack
 
 # start in the spack root directory
-cd $SPACK_ROOT
+cd "$SPACK_ROOT"
 
 # Run bash tests with coverage enabled, but pipe output to /dev/null
 # because it seems that kcov seems to undo the script's redirection
 if [ "$BASH_COVERAGE" = true ]; then
-    ${QA_DIR}/bashcov ${QA_DIR}/setup-env-test.sh &> /dev/null
+    "$QA_DIR/bashcov" "$QA_DIR/setup-env-test.sh" &> /dev/null
+    "$QA_DIR/bashcov" "$QA_DIR/completion-test.sh" &> /dev/null
 fi
 
 # run the test scripts for their output (these will print nicely)
-bash ${QA_DIR}/setup-env-test.sh
-zsh  ${QA_DIR}/setup-env-test.sh
-dash ${QA_DIR}/setup-env-test.sh
+bash "$QA_DIR/setup-env-test.sh"
+zsh  "$QA_DIR/setup-env-test.sh"
+dash "$QA_DIR/setup-env-test.sh"
+
+bash "$QA_DIR/completion-test.sh"