In the future we can expect many more matchers, so let's give them
a place to live.
Also moved matcher-related internal files to `internal` subfolder.
Ideally we should sort out all of our source code, but that will
have to come later.
This commit also forbids composing lvalues of composed matchers, as
per previous deprecation notice. I do not expect this to be contentious
in practice, because there was a bug in that usage for years, and
nobody complained.
b77cec05c0 fixed this problem for tagging tests, so that a test
case tagged with `[.foo]` would be parsed as tagged with `[.][foo]`.
This does the same for the test spec parsing.
Fixes#1798
Now that Catch2 is a proper library, we can always build the full
library (comparatively minor slowdown) and the user can avoid
including benchmarking headers to avoid the compilation slowdown.
Now that the recommended distribution and usage method is proper
library, users can just avoid including the matcher headers to get
basically the same effect.
