[cmake] refactor: shared/static handling

[crueter]

This significantly redoes the way shared and static libraries are
handled. Now, it's controlled by two options: SPIRV_TOOLS_BUILD_STATIC
and SPIRV_TOOLS_BUILD_SHARED.

The default configuration (no BUILD_SHARED_LIBS set, options left at
default) is to build shared ONLY if this is the master project, or
static ONLY if this is a subproject (e.g. FetchContent, CPM.cmake). Also
I should note that static-only (i.e. no shared) is now a supported
target, this is done because projects including it as a submodule e.g.
on Android or Windows may prefer this.

Now the shared/static handling:

• static ON, shared OFF: Only generates .a libraries.
• static ON, shared ON: Generates .a libraries, but also
  libSPIRV-Tools.so
• static OFF, shared ON: Only generates .so libraries.

Notable TODOs:

• SPIRV-Tools-shared.pc seems redundant--how should we handle which one
  to use in the case of distributions that distribute both types (MSYS2
  for instance)?
  • Note: pkgconfig sucks at this and usually just leaves it up to the
    user, so the optimal solution may indeed be doing absolutely
    nothing. CMake is unaffected :)
• use namespaces in the CMake config files pleaaaaase

This is going to change things a good bit for package maintainers, but
cest la vie. It's for the greater good, I promise.

Signed-off-by: crueter [email protected]