Change the behavior of python_fix_shebang to always output full path
to the Python interpreter (i.e. ${PYTHON}) instead of mangling
the original path. Most importantly, this ensures that:

1. EPREFIX is included in the final path

2. /usr/bin/env is replaced by the absolute path to avoid issues
when calling system executables from inside a venv

Note that this implies that a few unlikely corner cases may stop
working, notably:

a. "weird" shebangs such as "/usr/bin/foo python" will no longer work

b. the mangled scripts will escape temporary venv e.g. created
in distutils-r1 PEP517 mode (python_fix_shebang is not used in such
a way in ::gentoo)

Signed-off-by: Michał Górny <mgorny@gentoo.org>
---
eclass/python-utils-r1.eclass | 134 ++++++++++++--------------------
eclass/tests/python-utils-r1.sh | 86 ++++++++++++--------
2 files changed, 106 insertions(+), 114 deletions(-)

diff --git a/eclass/python-utils-r1.eclass b/eclass/python-utils-r1.eclass index 7b0c9aa280d8..98cb49c95fd7 100644
--- a/eclass/python-utils-r1.eclass
+++ b/eclass/python-utils-r1.eclass
@@ -1001,25 +1001,30 @@ _python_wrapper_setup() {
# @FUNCTION: python_fix_shebang
# @USAGE: [-f|--force] [-q|--quiet] <path>...
# @DESCRIPTION:
-# Replace the shebang in Python scripts with the current Python
-# implementation (EPYTHON). If a directory is passed, works recursively
-# on all Python scripts.
+# Replace the shebang in Python scripts with the full path
+# to the current Python implementation (PYTHON, including EPREFIX).
+# If a directory is passed, works recursively on all Python scripts
+# found inside the directory tree.
#
-# Only files having a 'python*' shebang will be modified. Files with
-# oth