Feature request: allow specifying alternative directory for deps.jl

For example via an environment variable. Here is an example of [the implementation](https://github.com/jlapeyre/PyCall.jl/commit/14aab392b54b1a0fa17835027f41cea81d482b84)

Then, if the environment variable is set, building the module writes `deps.jl` to a specified directory. And on loading, `deps.jl` is read from the same directory.

Together with [`CachePath.jl`]( https://github.com/jlapeyre/CachePath.jl) this avoids the problem of being required to rebuild `PyCall.jl` when you switch `libpython`, or its location.

`CachePath.jl` allows writing and reading a `.ji` file from a specified directory.

This [discourse post](https://discourse.julialang.org/t/cachepath-jl-allows-using-pycall-with-two-libpythons-and-no-recompilation/76325) gives an example of how this works.

Read the linked PyCall.jl implementation, then trace where the module build writes deps.jl and where loading reads it. Done means an environment variable selects the same alternative directory for both operations, supporting separate libpython locations without rebuilding; no test file is named in the issue.

Vom Indexierungsmodell aus dem Issue-Text verfasst.