This isn't needed, so just remove it.

This adds more checks to ensure that plugins aren't using the engine
from threads. This cause some issues in the code-index that need to be
fixed.

Recent language servers added a URI variant for this, deprecating the
rootPath path.

This tries a bit harder to ensure we have valid state going forward.

Ensure we don't have things accessing this from other threads, since it is
not safe to do so.

This is a a better type instance check since we use the actual value from
the hash table.

This allows the user to specify CMAKE=/path/to/cmake in case they need to
use an alternate cmake than the default cmake in $PATH.

This uses a fairly common convention to use a "run" target in the Makefile
to run the project. We still require that there is an install target
because the run manager requires that the project is installed before it
can be run.

We want to allow PATH to be overridden so that new PATH parts
can be appended. Such is used by SDK extensions.

If the builddir attribute is set, we need to propagate it to the
build configuration so the build system can potentially override
the build directory that will be used.

We don't need a local cancellable.

Special case, would be nice if this was abstracted but for now
we can just use the _build as convention.

This is useful in situations where we need to getenv, append,
setenv.

This is useful to clear existing values.

If the build configuration requires that the build is in tree,
then use _build inside the source directory.

This can be needed sometimes, such as some meson scripts in the
wild that integrate with Cargo.

This allows the build configuration to specify if it needs to
have in or out of tree builds. Flatpak can use this to propagate
the value of "builddir".

This is useful so that we can plumb support for modifying
$PATH when executing commands.

Instead of installing just a specific runtime, we want to
install the runtime, sdk, and any sdk-extension specified in the
GbpFlatpakConfiguration (manifest).

Related #345

This allows us to avoid setting NULL => master which is needed
to match SDK-extensions which don't necessarily have a version
identifier specified.

This uses the new API to ensure the runtimes for a given
configuration. This is more robust because it allows runtime
providers to install things like sdk-extensions.

This allows systems to ensure all the basic dependencies are
installed before using a runtime. A flatpak runtime provider
would use this to ensure that sdk-extensions are installed.

Related #345

This is needed so that we can install dependencies beyond that
of the runtime/sdk.

Related #345

This allows trying to locate any match when branch is NULL.