Warn for existing_rules

[laurentlb]

We've found this pattern in user code in Bazel:

    if foo in native.existing_rules():

It should be replaced with the much more efficient:

    if native.existing_rule(foo):

(also with not in instead of in)

cc @alandonovan

[alandonovan]

existing_rule is much more efficient than existing_rules, but still much less efficient than necessary: most users compute 'x in existing_rules()', which can be simplified to bool(existing_rule(x)), but this could be further simplified to 'rule_exists(x)', or alternatively a variant of existing_rule that doesn't materialize the fields until you ask for them. (For example, it could return a struct whose field names are attributes, analogous to ctx.attrs.)

It would be nice to introduce that feature before advising everyone change their code, only to change it again.

[laurentlb]

Also native.existing_rules()[x] can be simplified as native.existing_rule(x)

[tetromino]

Note that once https://github.com/bazelbuild/proposals/blob/main/designs/2021-06-15-improving-native.existing_rules.md is implemented, if foo in native.existing_rules() would be nearly as efficient as if native.existing_rule(foo).