gh-105931: Fix surprising compileall stripdir behaviour (GH-108671)

Before, the '-s STRIPDIR' option on
compileall lead to some surprising results as it only strips away
path components that match, but leaves alone the non-matching ones
interspersed in between.

For example, with: python -m compileall -s/path/to/another/src
/path/to/build/src/file.py

The resulting written path will be: build/file.py

This fix only strips directories that are a fully matching prefix of the
source path. If a stripdir is provided that is not a valid prefix, a
warning will be displayed (which can be silenced with '-qq').