internal/fetch: rename directory.go to unit.go

Change-Id: Iae06d3363e3565ef384eb87c0d25297f10e96792
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/256308
Trust: Julie Qiu <julie@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
diff --git a/internal/fetch/directory.go b/internal/fetch/unit.go
similarity index 100%
rename from internal/fetch/directory.go
rename to internal/fetch/unit.go
diff --git a/internal/fetch/directory_test.go b/internal/fetch/unit_test.go
similarity index 100%
rename from internal/fetch/directory_test.go
rename to internal/fetch/unit_test.go