cmd/gofmt: always format non-directory arguments again

golang.org/cl/284138 introduced a regression: running "gofmt foo" would
silently ignore the file due to its lack of a ".go" extension, whereas
the tool is documented otherwise:

	Given a file, it operates on that file; given a directory, it
	operates on all .go files in that directory, recursively.

This wasn't caught as there were no tests for these edge cases. gofmt's
own tests are regular Go tests, so it's hard to test it properly without
adding an abstraction layer on top of func main.

Luckily, this kind of test is a great fit for cmd/go's own script tests,
and it just takes a few straightforward lines.

Finally, add the relevant logic back, with documentation to clarify its
intentional purpose.

Fixes #45859.

Change-Id: Ic5bf5937b8f95fcdad2b6933227c8b504ef38a82
Reviewed-on: https://go-review.googlesource.com/c/go/+/315270


Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
Trust: Daniel Martí <mvdan@mvdan.cc>
Trust: Robert Griesemer <gri@golang.org>
Trust: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>