Julia doesn't find packages in depot_path anymore

I have a problem with using packages in Julia. It has worked before, and I'm not really sure why this has changed or how to troubleshoot.

I have a folder
/my_path/julia/packages
with Julia packages. For example, there is a folder
/my_path/julia/packages/FFTW/
with the FFTW package.
Further, I have changed the depot path to point at this directory by assigning JULIA_DEPOT_PATH before starting julia, so that

Base.DEPOT_PATH = ["/my_path/julia/"]

However, if I run
julia> using FFTW
I get the following error message:
>ERROR: ArgumentError: Package FFTW not found in current path:

• Run `import Pkg; Pkg.add("FFTW")` to install the FFTW package.

Any idea how I can troubleshoot or fix this?

Manipulating Base.DEPOT_PATH does not seem like a good idea.
The code proposed by @cmc will does not work (at least on Julia 1.3.1):

julia> Base.DEPOT_PATH = ["/some/path"]
ERROR: cannot assign variables in other modules

There is a workaround:

Base.DEPOT_PATH[1] = "/some/path"

However, the correct way is to assign the JULIA_DEPOT_PATH system variable before starting Julia, Windows:

set JULIA_DEPOT_PATH=c:\some\path

or

set JULIA_DEPOT_PATH=c:\some\path1;c:\some\path2

Linux/OSX:

export JULIA_DEPOT_PATH=/some/path

or

export JULIA_DEPOT_PATH=/some/path1:/some/path2

Unless you have a specific reason to do so (and if this is the case I'd be interested to hear it!), you don't need to fiddle with the DEPOT_PATH or LOAD_PATH variables: using Julia's package manager should be enough to cover your needs most of the time.

In this specific instance, have you tried to do what the error message suggests?

julia> import Pkg
julia> Pkg.add("FFTW")

LOAD_PATH, not DEPOT_PATH, will modify code loading.

You want to do something like push!(LOAD_PATH, /my_path/julia/packages).

I will echo @ffevotte and strongly suggest to not modify LOAD_PATH unless necessary. The benefits of organizing dependencies into Pkg environments far outweigh the small overhead of declaring them explicitly through Pkg.add.