configure: create a python venv unconditionally

This patch changes the configure script so that it always creates and
uses a python virtual environment unconditionally.

Meson bootstrapping is temporarily altered to force the use of meson
from git or vendored source (as packaged in our source tarballs). A
subsequent commit restores the use of distribution-vendored Meson.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-Id: <20230511035435.734312-16-jsnow@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

configure

@@ -625,7 +625,6 @@ check_py_version() {
 python=
 first_python=
 if test -z "${PYTHON}"; then
-    explicit_python=no
     # A bare 'python' is traditionally python 2.x, but some distros
     # have it as python 3.x, so check in both places.
     for binary in python3 python python3.11 python3.10 python3.9 python3.8 python3.7 python3.6; do
@@ -644,7 +643,6 @@ else
     # Same as above, but only check the environment variable.
     has "${PYTHON}" || error_exit "The PYTHON environment variable does not point to an executable"
     python=$(command -v "$PYTHON")
-    explicit_python=yes
     if check_py_version "$python"; then
         # This one is good.
         first_python=
@@ -729,7 +727,7 @@ for opt do
   ;;
   --install=*)
   ;;
-  --python=*) python="$optarg" ; explicit_python=yes
+  --python=*) python="$optarg"
   ;;
   --skip-meson) skip_meson=yes
   ;;
@@ -1090,8 +1088,34 @@ if ! check_py_version "$python"; then
       "Use --python=/path/to/python to specify a supported Python."
 fi
 
-# Resolve PATH + suppress writing compiled files
-python="$(command -v "$python") -B"
+# Resolve PATH
+python="$(command -v "$python")"
+explicit_python=yes
+
+# Create a Python virtual environment using our configured python.
+# The stdout of this script will be the location of a symlink that
+# points to the configured Python.
+# Entry point scripts for pip, meson, and sphinx are generated if those
+# packages are present.
+
+# Defaults assumed for now:
+# - venv is cleared if it exists already;
+# - venv is allowed to use system packages;
+# - all setup is performed **offline**;
+# - No packages are installed by default;
+# - pip is not installed into the venv when possible,
+#   but ensurepip is called as a fallback when necessary.
+
+echo "python determined to be '$python'"
+echo "python version: $($python --version)"
+
+python="$($python -B "${source_path}/python/scripts/mkvenv.py" create pyvenv)"
+if test "$?" -ne 0 ; then
+    error_exit "python venv creation failed"
+fi
+
+# Suppress writing compiled files
+python="$python -B"
 
 has_meson() {
   local python_dir=$(dirname "$python")