Remove build-support/virtualenv; use stdlib venv.

Since pants now only runs on Python 3.6+ we can rely on `python -mvenv`
being available and simplify venv creation in our support scripts.

[ci skip-rust-tests]

.gitignore

@@ -38,8 +38,6 @@ out/
 /bin/
 /build-support/*.pex
 /build-support/virtualenvs
-/build-support/virtualenv-*
-/build-support/virtualenv.dist/
 /build-support/phab/
 /build-support/twine-deps.venv/
 /dist/

build-support/bin/release.sh

@@ -177,8 +177,8 @@ function activate_tmp_venv() {
   # place, Shellcheck will not be able to find it so we tell Shellcheck to ignore the file.
   # shellcheck source=/dev/null
   VENV_DIR=$(mktemp -d -t pants.XXXXX) && \
-  "${ROOT}/build-support/virtualenv" "$VENV_DIR" && \
-  source "$VENV_DIR/bin/activate"
+  "${PY}" -mvenv "${VENV_DIR}" && \
+  source "${VENV_DIR}/bin/activate"
 }
 
 function pre_install() {
@@ -300,7 +300,7 @@ function activate_twine() {
   local -r venv_dir="${ROOT}/build-support/twine-deps.venv"
 
   rm -rf "${venv_dir}"
-  "${ROOT}/build-support/virtualenv" "${venv_dir}"
+  "${PY}" -mvenv "${venv_dir}"
   # Because the venv/bin/activate script's location is dynamic and not located in a fixed
   # place, Shellcheck will not be able to find it so we tell Shellcheck to ignore the file.
   # shellcheck source=/dev/null

build-support/bin/shellcheck.py

@@ -27,12 +27,10 @@ def run_shellcheck() -> None:
     targets = set(glob("./**/*.sh", recursive=True)) | {
         "./pants",
         "./build-support/pants_venv",
-        "./build-support/virtualenv",
         "./build-support/githooks/pre-commit",
         "./build-support/githooks/prepare-commit-msg",
     }
     targets -= set(glob("./build-support/bin/native/src/**/*.sh", recursive=True))
-    targets -= set(glob("./build-support/virtualenv.dist/**/*.sh", recursive=True))
     targets -= set(glob("./build-support/twine-deps.venv/**/*.sh", recursive=True))
     command = ["shellcheck", "--shell=bash", "--external-sources"] + sorted(targets)
     try:

build-support/pants_venv

@@ -33,7 +33,7 @@ function remove_venv() {
 
 function create_venv() {
   remove_venv
-  "${REPO_ROOT}/build-support/virtualenv" "$(venv_dir)"
+  "${PY}" -mvenv "$(venv_dir)"
 }
 
 function ensure_gcc() {