Don't silently ignore directories in source-include

[woodruffw]

Summary

WIP.

This is a stab at fixing #16751. The basic observation here is that a user who tries to include something like tests/ almost certainly means tests/**, so we use the same logic as in source_dist_matcher and append **.

Closes #16751.

Test Plan

Added two new tests for this: one that ensures we transform tests/ correct and another that ensures we warn when the user passes tests (i.e. without the trailing slash).

[woodruffw]

Okay, I simplified this a bit -- both foo and foo/ get transformed into foo/** if foo is a directory now, which aligns with our pre-existing semantics around how single files behave.

(I've also added a test for that single-file include behavior, since I couldn't find a pre-existing one.)