build info should be available in early bound funcs

Early bound funcs can access a `building` boolean, but if True, there is no further access to build info (such as install path).

We should bind a new object, which is the same as the one in the upcoming 'pre_build_commands' feature: https://github.com/nerdvegas/rez/pull/825/files#diff-a40d02fd42f11a3aa670f277775c4f5eR228-R257

Start with the early bound function handling and inspect the build-info object used by the referenced pre_build_commands feature in pull request #825. Confirm how the existing building boolean is bound and identify the corresponding build details, such as the install path. Done means early bound functions can access that object and its build information while building.

Written by the indexing model from the issue text.