Clean up the `ProgramRunner` class:
* use llvm-provided wrappers and system-agnostic variables to compute
the `Paths` variable
* pre-compute the list of path variables so it's not re-computed on
every invocation of `run` or `isProgramAvailable`
* Wrap the logging code so that it's not triggered if the logger is
disabled
We used to collect all binaries into the `bin/` directory. However this
led to confusions since certain commands where available both as
`revng-command` and `revng command`.
This commit moves all the executables except `revng` into
`libexec/revng`, which, according to FHS, is dedicated to "internal
binaries that are not intended to be executed directly by users or shell
scripts".
`main` is not in all cases a dynamically exported symbol, therefore,
it's not safe to rely on it.
This commit switches to use `PathList`'s `getCurrentExecutableFullPath`,
which reads `/proc/self/exe`.
Giacomo Vercesi