Desugar the locals in object comprehensions
"traditionally" instead of handling them manually.

Object comprehensions allow the locals to depend
on the index variable which means that they are separate
for each field. It doesn't make sense to treat them as
a property of the whole object.

Fixes bazel-contrib#358.

add missing docstrings for rules_python_external

also remove unused attr on pip_repository repo rule

…el-contrib#358)

This PR adds support for built-in conversion functions, such as
`string(x)`, used as index expr. Currently, without this support,
NilAway reports a false positive for the example below.
```
func test(v uint8) {
		m := make(map[string]*int)
		if m[string(v)] != nil {
			_ = *m[string(v)]  // false positive was reported here
		}
}
```

Go compiler doesn't treat `string(x)` as a call to a built-in function
named “string”, but instead it's considered to be a conversion using the
predeclared type string. Therefore, our existing logic of
`*types.BuiltIn` did not cover this case. We had to add special support
for this case by checking for
`pass.TypesInfo.ObjectOf(ident).Type().(*types.Basic)`.

[Closes bazel-contrib#357 ]