Migrate away from sp-grep (#1119)

Switches the last consumers of it to a simpler utility function that
uses `pkgutil.find_loader()`:
https://docs.python.org/3/library/pkgutil.html#pkgutil.find_loader

Both of these consumers are covered by existing tests.

Then removes `sp-grep` and the remaining parts of `pip-pop`.

Closes @W-8208817@.

bin/compile

@@ -68,9 +68,9 @@ DEFAULT_PYTHON_STACK="heroku-18"
 WARNINGS_LOG=$(mktemp)
 RECOMMENDED_PYTHON_VERSION=$DEFAULT_PYTHON_VERSION
 
-# The buildpack ships with a few executable tools (e.g. pip-grep, etc).
+# The buildpack ships with a few executable tools.
 # This installs them into the path, so we can execute them directly.
-export PATH=$PATH:$ROOT_DIR/vendor/:$ROOT_DIR/vendor/pip-pop
+export PATH=$PATH:$ROOT_DIR/vendor/
 
 # Set environment variables if they weren't set by the platform.
 # Note: this is legacy, for a deprecated build system known as Anvil.

bin/steps/collectstatic

@@ -20,8 +20,8 @@ MANAGE_FILE=${MANAGE_FILE:-fakepath}
 # Legacy file-based support for $DISABLE_COLLECTSTATIC
 [ -f .heroku/collectstatic_disabled ] && DISABLE_COLLECTSTATIC=1
 
-# Ensure that Django is explicitly specified in requirements.txt
-sp-grep -s django && DJANGO_INSTALLED=1
+# Ensure that Django is actually installed.
+is_module_available 'django' && DJANGO_INSTALLED=1
 
 
 if [ ! "$DISABLE_COLLECTSTATIC" ] && [ -f "$MANAGE_FILE" ] && [ "$DJANGO_INSTALLED" ]; then

bin/steps/nltk

@@ -14,7 +14,7 @@
 source "$BIN_DIR/utils"
 
 # Check that nltk was installed by pip, otherwise obviously not needed
-if sp-grep -s nltk; then
+if is_module_available 'nltk'; then
     puts-step "Downloading NLTK corpora…"
 
     nltk_packages_definition="$BUILD_DIR/nltk.txt"

bin/utils

@@ -96,3 +96,11 @@ python_sqlite3_check() {
   ( python2_check "$VERSION" && version_gte "$VERSION" "$MIN_PYTHON_2" ) \
     || ( python3_check "$VERSION" && version_gte "$VERSION" "$MIN_PYTHON_3" )
 }
+
+is_module_available() {
+  # Returns 0 is the specified module exists, otherwise returns 1.
+  # Uses pkgutil rather than pkg_resources or pip's CLI, since pkgutil exists
+  # in the stdlib, and doesn't depend on the choice of package manager.
+  local module_name="${1}"
+  python -c "import sys, pkgutil; sys.exit(0 if pkgutil.find_loader('${module_name}') else 1)"
+}