[PASS][RELAY] polish pass infra

[tqchen]

Set of change I made when reading the pass infra code. Some of them should have been caught during code reviews.

Summary of code changes

• It is wasteful to create a unordered_set every time and then check, just use a linear scan will have the same time complexity and should be fine if the set is small.
• PassEnabled is not quite right(given that it uses GetPass to create a pass just to get the level)
• Move dependency resolving into the sequential(which manages the passes).

Additional changes

• Use PackedFunc for Pass lookup

cc @zhiics

[zhiics]

LGTM. Thanks for improvement.