Externals Update, master branch (2019.09.06.)

This MR, together with !26327 (merged), is supposed to completely replace !26168 (closed).

First off, the update switches the externals from atlasexternals-2.0.37 to atlasexternals-2.0.39. The full list of changes is a bit long (atlasexternals@2.0.37...2.0.39)...

• Fixed the setup/usage of the $TDAQ_RELEASE_BASE environment variable in Findtdaq.cmake and Findtdaq-common.cmake (ATLINFR-3098);
• Updated the standalone builds to LHAPDF 6.2.1 (needed for C++17 compatibility with Clang);
• Updated the standalone builds to MCUtils 1.3.3. See for instance !26148 (closed) for some more info on this.
• Added a flake8 checker for ATLAS copyright statements in Python files (courtesy of @fwinkl);
• Made each CMake (re-)configuration delete the project's main .rootmap file (ATLASDQ-730);
• Updates to CheckerGccPlugins (courtesy of @ssnyder);
• Introduced a mechanism for preventing linking against component libraries, one putting statements into the compilation/link commands that would make those fail (ATLINFR-3143);
• Hid the "Package build suceeded" messages when per-package log file writing is disabled.

It's the update for ATLINFR-3143 that made all the other CMake updates necessary. These were mistakes in our configuration that !26168 (closed) did not catch.