Don't create and remove temp dirs unnecessarily when they are not used,
looks like these were just copy-pastes without intended side-effects.

Avoid an unnecessary call to os.path.join().

Previously, include a directory result in an error like this:

    mydir is a directory. bst command expects a .bst file.

Note that the file containing the include was not mentioned.

Now we get an error like this instead:

    element.bst [line 12 column 0]: Include block references a
    directory instead of a file: 'mydir'.

Previously, a missing include would result in an error like this:

    Could not find file at not-a-file.include

Note that the file containing the include was not mentioned.

Now we get an error like this instead:

    element.bst [line 7 column 5]: Include block references a file that
    could not be found: 'not-a-file.include'.

Use the provenance of the include block, instead of the whole node.

contributing: snakeviz replaces pyflame+flamegraph

See merge request BuildStream/buildstream!1129

Replace the instructions for pyflame+flamegraph with simpler ones for
snakeviz. For our general use-case this seems to be easier and better.
Usage of this tool was demonstrated at the 2019 BuildStream Gathering in
January by Daniel Silverstone, when presenting the aggregate results of
profiling on many target environments.

Here is the relevant mailing list thread:

"Profiling before the gathering"
https://mail.gnome.org/archives/buildstream-list/2019-January/msg00057.html

Symlink fixes

See merge request BuildStream/buildstream!1138

This matches the change in utils.list_relative_paths() that now sorts
all symlinks as files, instead of following the broken behavior of
os.walk().