Allow `load` to be optional

### Description of the feature request:

It is sometimes useful to optionally load files if they are present. This can be seen in `.bazelrc`'s `try-import` statement. It would be very helpful if there was a similar `try-load` statement that could also be called (or an `optional` flag to the existing `load` statement.

Places where this might be useful include in macro generation, where a `try-load` could be attempted, and if successful, additional flags could be set on generated targets.

Start by examining the existing `load` behavior and the `.bazelrc` `try-import` statement for relevant semantics. Decide whether the feature should use a `try-load` statement or an optional flag, then identify the associated tests and define completion as allowing absent files without failing while preserving normal loading behavior.

Written by the indexing model from the issue text.