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.