[move package test] panics when cache is hot

[vgao1996]

Right now if you run move package test inside a package that depends on MoveStdlib and MoveStdlib only, for the first time it'll succeed:

BUILDING MoveStdlib
BUILDING Foo
Running Move unit tests
[ PASS    ] 0x1::A::one_plus_one
[ PASS    ] 0x1::A::should_panic
[ PASS    ] 0x1::A::test_with_arg
Test result: OK. Total tests: 3; passed: 3; failed: 0

However if you run this command for the second time, it complains about not being able to find a source file:

CACHED MoveStdlib
BUILDING Foo
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: No such file or directory './build/MoveStdlib/sources/ASCIITests.move'', language/tools/move-cli/src/package/cli.rs:510:83