Log Message:
pkgtools/pkglint: update to 21.4.1

Changes since 21.4.0:

Running 'pkglint doc/CHANGES-2021' now warns about issues for this
single file.  Previously, it was necessary to specify '-Cglobal' as
well, but then pkglint also warned about issues in all other CHANGES
files.

Pkglint no longer warns about the characters '!' and '@' in
GO_MODULES_FILES, since these are legitimate.  Fixes PR pkg/56595.

Small cleanups in the pkglint testing infrastructure.