Allow using files from the pool while importing source packages

Sometimes source packages reference files already present in the pool.

Allow for those file to be omitted when importing packages either via
`repo add` or `repo include`. If file is missing, aptly would make
an attempt to look up file in the package pool (by checksum) and
use it.

Fixes: #278

deb/import.go

@@ -147,14 +147,34 @@ func ImportPackageFiles(list *PackageList, packageFiles []string, forceReplace b
 		// go over all the other files
 		for i := range files {
 			sourceFile := filepath.Join(filepath.Dir(file), filepath.Base(files[i].Filename))
-			files[i].PoolPath, err = pool.Import(sourceFile, files[i].Filename, &files[i].Checksums, false, checksumStorage)
+
+			_, err = os.Stat(sourceFile)
+			if err == nil {
+				files[i].PoolPath, err = pool.Import(sourceFile, files[i].Filename, &files[i].Checksums, false, checksumStorage)
+				if err == nil {
+					candidateProcessedFiles = append(candidateProcessedFiles, sourceFile)
+				}
+			} else if os.IsNotExist(err) {
+				// if file is not present, try to find it in the pool
+				var (
+					err2  error
+					found bool
+				)
+
+				files[i].PoolPath, found, err2 = pool.Verify("", files[i].Filename, &files[i].Checksums, checksumStorage)
+				if err2 != nil {
+					err = err2
+				} else if found {
+					// clear error, file is already in the package pool
+					err = nil
+				}
+			}
+
 			if err != nil {
 				reporter.Warning("Unable to import file %s into pool: %s", sourceFile, err)
 				failedFiles = append(failedFiles, file)
 				break
 			}
-
-			candidateProcessedFiles = append(candidateProcessedFiles, sourceFile)
 		}
 		if err != nil {
 			// some files haven't been imported