Add ability to specify recipes in separate files

This can now be achieved with the new `recipe` directive:

recipe <language> <file>

Note that similar to the use of if-else and switch directives with recipes,
this directive requires explicit % recipe header. For example, instead of:

file{foo.output}:
{{
   echo 'hello' >$path($>)
}}

We can now write:

file{foo.output}:
%
recipe buildscript hello.buildscript

With hello.buildscript containing:

echo 'hello' >$path($>)

Similarly, for C++ recipes (this time for a pattern), instead of:

[rule_name=hello] file{~'/(.+)\.output/'}:
% update clean
{{ c++ 1 --

--

...

}}

We can now write:

[rule_name=hello] file{~'/(.+)\.output/'}:
% update clean
recipe c++ hello.cxx

With hello.cxx containing:

// c++ 1 --

--

...

Relative <file> paths are resolved using the buildfile directory that contains
the `recipe` directive as a base.

Note also that this mechanism can be used in exported buildfiles with recipe
files placed into build/export/ together with buildfiles.

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