Feature request: environmentally determined load paths

Nobody has claimed this yet.

@indygreg closed #328 by committing 2dbe0d2, thereby adding support for Starlark's [`load`] statement. This makes it easier to share target definitions, leaving minimal configuration to downstream projects. A major difficulty remains in using the shared target definitions: we **have to hard code the locations of the shared `.bzl` files**. I'll explain with a schematic description of my intended workflow.

# Example use case

My Python library `pylib` defines `pylib/pylib.bzl`, distributing it as package data so that it's installed and available when my coworkers install `pylib` with Pip. Let's say my coworkers are developing `pyapp`, which requires `pylib`. `pyapp` will be distributed to customers as a pyoxidized `pyapp.exe` generated according to defaults set in `pylib.bzl`. According to `pylib.bzl`'s API, all they need to do is write the following in their `pyoxidizer.bzl`. (See "Current workaround" at indygreg/PyOxidizer#331 for an explanation of passing `CWD` and `BUILD_TARGET_TRIPLE`.)

```bzl
def project_settings():
return dict(
project_name="pyapp",
foo="bar",
# See "Current workaround" at https://github.com/indygreg/PyOxidizer/issues/331
BUILD_TARGET_TRIPLE=BUILD_TARGET_TRIPLE, CWD=CWD,
)
register_target("project_settings", project_settings)
load("venv/lib/python3.8/site-packages/pylib/pylib.bzl", "pylib_install")
resolve_targets()
```

The problematic line is the [`load`] statement. If one person names their virtual environment something other than `venv`, they're already in trouble. I can (and do) solve that particular issue by using [Tox](https://tox.readthedocs.io/en/latest/). But on Windows the `site-packages` directory is in `Lib\site-packages` rather than `lib/python3.X/site-pacakges`. The essential issue is that [`load`] **requires its first argument be a string literal**, so you can't compute that load path.

`pyapp` could vendor `pylib.bzl` and change the [`load`] line to
```bzl
load("vendor/pylib/pylib.bzl", "pylib_install")
```
But then obtaining and upgrading `pylib.bzl` is no longer as easy as Pip installing pylib.

# Potential solutions

A quick and easy option would be to perform environment variable expansion on the `load` path. I would suggest requiring the environment variable name to be surrounded by `$(` and `)` or `${` and `}` [as in Make](https://www.gnu.org/software/make/manual/make.html#Reference) ([Pip supports](https://pip.pypa.io/en/stable/reference/pip_install/#id10) only the latter; [Bazel supports](https://docs.bazel.build/versions/3.7.0/be/make-variables.html#use) only the former; `ProgramFiles(x86)` is a commonly defined environment variable on Windows, but both POSIX and Windows support almost any environment variable name not containing a literal `=`).

A step further is Bazel's ["Make" variables](https://docs.bazel.build/versions/3.7.0/be/make-variables.html) expansion. Bazel's own solution (though going beyond what I would need) would be to make the first argument to [`load`] be a [label](https://docs.bazel.build/versions/3.7.0/build-ref.html#labels).

**If you're down with initially just offering environment variable expansion, I can take a crack at a PR.**

[`load`]: https://github.com/bazelbuild/starlark/blob/master/spec.md#load-statements

The issue names no implementation files or tests. Start by locating the Starlark `load` handling and its existing test coverage, then compare the proposed environment-variable expansion forms and cross-platform path behavior. Done means an agreed syntax is implemented, documented, and covered for POSIX and Windows paths.

Written by the indexing model from the issue text.