[SV 66273] Double-colon targets must not be intermediate

An explicitly mentioned double-colon target cannot be intermediate.

* src/read.c (record_files): Set file->is_explicit in the case of an
explicitly mentioned file built by a double colon rule.
* src/file.c (print_file): Have print_file print whether a file is
explicitly mentioned.
* tests/scripts/features/patternrules: Add tests.