internal/pkg: garbage collection

This destroys cache entries not referred to by user-specified artifacts and optionally their inputs.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/pkg/pkg_test.go

@@ -486,7 +486,17 @@ func checkWithCache(t *testing.T, testCases []cacheTestCase) {
 					tc.early(t, base)
 				}
 				tc.f(t, base, c)
-				scrubFunc = func() error { return c.Scrub(1 << 7) }
+				scrubFunc = func() error {
+					err = c.Scrub(1 << 7)
+					idents, checksums, cleanErr := c.Clean(false, false)
+					if len(idents) > 0 {
+						t.Errorf("destroyed %d idents", len(idents))
+					}
+					if len(checksums) > 0 {
+						t.Errorf("destroyed %d checksums", len(checksums))
+					}
+					return errors.Join(err, cleanErr)
+				}
 			}
 
 			var restoreTemp bool
@@ -561,10 +571,7 @@ func checkWithCache(t *testing.T, testCases []cacheTestCase) {
 			if err := pkg.HashDir(&checksum, base); err != nil {
 				t.Fatalf("HashDir: error = %v", err)
 			} else if checksum != want {
-				t.Fatalf("(scrubbed) HashDir: %v", &pkg.ChecksumMismatchError{
-					Got:  checksum,
-					Want: want,
-				})
+				t.Fatalf("(scrubbed) %s", expectsFrom(base.String()))
 			}
 		})
 	}