WIP: Externals Update, master branch (2019.08.28.)

This MR updates all projects to atlasexternals-2.0.38.

There are a few changes since atlasexternals-2.0.37, namely (atlasexternals@2.0.37...2.0.38):

• 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. While right now it seems that the new tag of MCUtils may have some issues, since we don't build standalone releases needing this external from master (or at least we're not interested in the physics performance of VP1Lite like that...), I decided not to undo this change (for now)...
• 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);
• Made all symbols in component libraries hidden, to prevent clients from linking against them (ATLINFR-3143).

It's the last one that may break a number of things... 😦 I'm starting a full private build of this update as well, but I wanted to open the MR at the same time, to let the CI do its job as well. We'll see how many libraries get broken because of the symbol hiding...