improve require deprecation

ref #13107

base/deprecated.jl

@@ -661,14 +661,36 @@ end
 
 ## require ##
 
+function maybe_require_file(name::AbstractString)
+    isabspath(name) && return name
+    isfile(name) && return abspath(name)
+    if !endswith(name,".jl")
+        fname = string(name,".jl")
+        isfile(fname) && return abspath(fname)
+    end
+    return name
+end
+
 include("require.jl")
 @noinline function require(f::AbstractString)
-    if !(endswith(f,".jl") || contains(f,path_separator))
-        # for require("Name") this function shouldn't be needed at all
-        error("use `using` or `import` to load packages")
-    end
     depwarn("`require` is deprecated, use `using` or `import` instead", :require)
-    OldRequire.require(f)
+    if endswith(f,".jl") || contains(f,path_separator)
+        # specifying file path
+        OldRequire.require(f)
+    else
+        # require("Foo") --- ambiguous. might be file or package
+        filename = maybe_require_file(f)
+        if filename == f
+            mod = symbol(require_modname(f))
+            M = current_module()
+            if isdefined(M,mod) && isa(eval(M,mod),Module)
+                return
+            end
+            require(mod)
+        else
+            OldRequire.require(f)
+        end
+    end
 end
 @noinline function require(f::AbstractString, fs::AbstractString...)
     require(f)

base/loading.jl

@@ -162,7 +162,7 @@ function __precompile__(isprecompilable::Bool=true)
     end
 end
 
-function require_filename(name::AbstractString)
+function require_modname(name::AbstractString)
     # This function can be deleted when the deprecation for `require`
     # is deleted.
     # While we could also strip off the absolute path, the user may be
@@ -192,7 +192,7 @@ function reload(name::AbstractString)
         error("use `include` instead of `reload` to load source files")
     else
         # reload("Package") is ok
-        require(symbol(require_filename(name)))
+        require(symbol(require_modname(name)))
     end
 end
 

base/require.jl

@@ -11,7 +11,7 @@ function find_in_path(name::AbstractString)
         name = string(base,".jl")
         isfile(name) && return abspath(name)
     end
-    for prefix in [Pkg.dir(), LOAD_PATH]
+    for prefix in [Pkg.dir(); LOAD_PATH]
         path = joinpath(prefix, name)
         isfile(path) && return abspath(path)
         path = joinpath(prefix, base, "src", name)