[bug] register does not work with the module being used as alias

[kunzaatko]

When using

Unitful.register(@__MODULE__)

the registered units, are linked to the module instead of the identifier. For this reason, the @u_str macro does not work when the module is imported with as.

MWE

With the module

module MM
using Unitful

function __init__()
    Unitful.register(@__MODULE__)
end

@unit px "px" Pixel 36u"nm" false
end

and in the REPL

In [1]: using MM: MM as M

In [2]: using Unitful

In [3]: u"px"
ERROR: LoadError: ArgumentError: Symbol `px` was found in the globally registered unit module MM
but was not in the provided list of unit modules Unitful.

(Consider `using MM` in your module if you are using `@u_str`?)
Stacktrace:
 [1] lookup_units(unitmods::Vector{Module}, sym::Symbol)
   @ Unitful ~/.julia/packages/Unitful/orvol/src/user.jl:701
 [2] var"@u_str"(__source__::LineNumberNode, __module__::Module, unit::Any)
   @ Unitful ~/.julia/packages/Unitful/orvol/src/user.jl:639
in expression starting at REPL[3]:1

but when you import it using the name where it is registered, it works as normal

 In [4]: using MM

 In [5]: u"px"
 > px