Symbol: Add SourceFileType source_type
Symbol: Add SourceFileType source_type

Pass this through from the SourceFileType of the source file.  If there
is no source file, use SourceFileType.NONE.

Redefine 'external_package' in terms of this new property.

kamanashisroy/aroop,tjol/vala-tjol,spidermonkdust/vala,playya/vala,GNOME/vala,playya/vala,rvu95/vala,flobrosch/vala-fixes,Distrotech/vala,playya/vala,Distrotech/vala,frida/vala,jessevdk/vala,frida/vala,frida/vala,severinh/vala,Distrotech/vala,frida/vala,jessevdk/vala,jessevdk/vala,spidermonkdust/vala,GNOME/vala,frida/vala,kamanashisroy/aroop,severinh/vala,flobrosch/vala-fixes,severinh/vala,rvu95/vala,kamanashisroy/aroop,tjol/vala-tjol,GNOME/vala,flobrosch/vala-fixes,xdave/pvala,rvu95/vala,spidermonkdust/vala,tjol/vala-tjol,GNOME/vala