pkgtools/pkglint: update to 20.1.16

Changes since 20.1.15:

When a package adds an additional version requirement for another
package, it must do so using BUILDLINK_API_DEPENDS instead of
BUILDLINK_ABI_DEPENDS.  Most packages already do this.

When a values is appended to an undefined variable using the += operator,
bmake does not add a space before, and pkglint caught up to do the same.
This change has no practical consequences though.

As always, a bit of refactoring.  The method names of MkAssignChecker
contained the redundant word "varassign".

(rillig)