The previously used regular expression tried to handle both exact
matches and and prefix matches in one go, using this approach:

    re.compile(r'^%s\.?' % pkg)

However, this is incorrect, since the literal dot is optional in the
pattern, causing longer matches to also get included. For example, ‘foo’
should match ‘foo’ and ‘foo.bar’, but it also incorrectly matches
‘foobar’:

    >>> re.compile(r'^foo\.?').match('foobar')
    <_sre.SRE_Match object; span=(0, 3), match='foo'>

In practice, a command like this (using the pytest plugin as an example)
is supposed to check the ‘flask’ package and any modules below it:

    pytest --typeguard-packages=flask

... but in reality it also checks other packages, such as
‘flask_sqlalchemy’ and ‘flask_redis’, if those happen to be installed.

This can be easily fixed by not using regular expression, but simple
string matching instead.