Add file:/// handling in download tool (#2635)

* Add file:/// handling in download tool

* Add Windows example and note that it's only for absolute paths

reference/conanfile/tools/files/downloads.rst

@@ -90,6 +90,8 @@ Parameters:
 
 - **conanfile** (Required): Conanfile object, use ``self`` always
 - **url** (Required): URL to download. It can be a list, which only the first one will be downloaded, and the follow URLs will be used as mirror in case of download error.
+                      Files accessible in the local filesystem can be referenced with a URL starting with ``file:///`` followed by an absolute path to a file
+                      (where the third ``/`` implies ``localhost``).
 - **filename** (Required): Name of the file to be created in the local storage
 - **verify** (Optional, Defaulted to ``True``): When False, disables https certificate validation.
 - **retry** (Optional, Defaulted to ``1``): Number of retries in case of failure.
@@ -127,9 +129,14 @@ Examples:
     # Download and check file checksum
     download(self, "http://someurl/somefile.zip", "myfilename.zip", md5="e5d695597e9fa520209d1b41edad2a27")
 
+    # Retrieve file that is visible in the local filesystem
+    tools.download("file:///C:/Users/MyUser/Downloads/somefile.zip", "myfilename.zip", md5="e5d695597e9fa520209d1b41edad2a27")
+    tools.download("file:///home/myuser/downloads/somefile.zip", "myfilename.zip", md5="e5d695597e9fa520209d1b41edad2a27")
+
     # to add mirrors
     download(self, ["https://ftp.gnu.org/gnu/gcc/gcc-9.3.0/gcc-9.3.0.tar.gz",
-                    "http://mirror.linux-ia64.org/gnu/gcc/releases/gcc-9.3.0/gcc-9.3.0.tar.gz"],
+                    "http://mirror.linux-ia64.org/gnu/gcc/releases/gcc-9.3.0/gcc-9.3.0.tar.gz",
+                    "file:///home/myuser/localmirror/gcc-9.3.0/gcc-9.3.0.tar.gz"],
                     "gcc-9.3.0.tar.gz",
                    sha256="5258a9b6afe9463c2e56b9e8355b1a4bee125ca828b8078f910303bc2ef91fa6")