Add a hook for callbacks when files are included.

This allows one to obtain the file dependencies of a non-precompiled package.

base/loading.jl

@@ -201,6 +201,10 @@ const package_locks = Dict{Symbol,Condition}()
 # Callbacks take the form (mod::Symbol) -> nothing.
 # WARNING: This is an experimental feature and might change later, without deprecation.
 const package_callbacks = Any[]
+# to notify downstream consumers that a file has been included into a particular module
+# Callbacks take the form (mod::Module, filename::String) -> nothing
+# WARNING: This is an experimental feature and might change later, without deprecation.
+const include_callbacks = Any[]
 
 # used to optionally track dependencies when requiring a module:
 const _concrete_dependencies = Any[] # these dependency versions are "set in stone", and the process should try to avoid invalidating them
@@ -519,6 +523,9 @@ end
 include_relative(mod::Module, path::AbstractString) = include_relative(mod, String(path))
 function include_relative(mod::Module, _path::String)
     path, prev = _include_dependency(_path)
+    for callback in include_callbacks # to preserve order, must come before Core.include
+        invokelatest(callback, mod, path)
+    end
     tls = task_local_storage()
     tls[:SOURCE_PATH] = path
     local result

test/loading.jl

@@ -31,7 +31,11 @@ end
 @test @nested_LINE_expansion() == ((@__LINE__() - 4, @__LINE__() - 12), @__LINE__())
 @test @nested_LINE_expansion2() == ((@__LINE__() - 5, @__LINE__() - 9), @__LINE__())
 
+loaded_files = String[]
+push!(Base.include_callbacks, (mod::Module, fn::String) -> push!(loaded_files, fn))
 include("test_sourcepath.jl")
+@test length(loaded_files) == 1 && endswith(loaded_files[1], "test_sourcepath.jl")
+pop!(Base.include_callbacks)
 thefname = "the fname!//\\&\1*"
 include_string_test_func = include_string(@__MODULE__, "include_string_test() = @__FILE__", thefname)
 @test include_string_test_func() == thefname