introduce a docfiledir

It essentially is to the typesetfiles what testfiledir is to the
testfiles: The files found therein are only introduced into the
unpacking process when typesetting the documentation.

l3build/l3build.dtx

@@ -88,6 +88,7 @@
 \luavarset{textfiles}       {\{"*.md", "*.txt"\}}{Plain text files to send to CTAN as-is.}
 \luavarset{typesetdemofiles}{\{~\}}       {Files to typeset before the documentation (as demos), but where the PDF results are not included in the release.}
 \luavarset{typesetfiles}    {\{"*.dtx"\}} {Files to typeset for documentation.}
+\luavarset{docfiledir}      {maindir .. "/doc"}{Where to look for the |typesetfiles|.}
 \luavarset{typesetsuppfiles}{\{~\}}       {Files needed to support typesetting when \enquote{sandboxed}.}
 \luavarset{unpackfiles}     {\{"*.ins"\}} {Files to run to perform unpacking.}
 \luavarset{unpacksuppfiles} {\{~\}}       {Files needed to support unpacking when \enquote{sandboxed}.}

l3build/l3build.lua

@@ -101,6 +101,7 @@ sourcefiles      = sourcefiles      or {"*.dtx", "*.ins"}
 textfiles        = textfiles        or {"*.md", "*.txt"}
 typesetdemofiles = typesetdemofiles or { }
 typesetfiles     = typesetfiles     or {"*.dtx"}
+docfiledir       = docfiledir       or maindir .. "/doc"
 typesetsuppfiles = typesetsuppfiles or { }
 unpackfiles      = unpackfiles      or {"*.ins"}
 unpacksuppfiles  = unpacksuppfiles  or { }
@@ -1859,7 +1860,7 @@ function doc(files)
     cp(i, supportdir, typesetdir)
   end
   depinstall(typesetdeps)
-  unpack()
+  unpack({".", docfiledir})
   -- Main loop for doc creation
   for _, typesetfiles in ipairs({typesetdemofiles, typesetfiles}) do
     for _,i in ipairs(typesetfiles) do
@@ -2068,12 +2069,12 @@ end
 
 -- Unpack the package files using an 'isolated' system: this requires
 -- a copy of the 'basic' DocStrip program, which is used then removed
-function unpack()
+function unpack(sourcedir)
   local errorlevel = depinstall(unpackdeps)
   if errorlevel ~= 0 then
     return errorlevel
   end
-  errorlevel = bundleunpack()
+  errorlevel = bundleunpack(sourcedir)
   if errorlevel ~= 0 then
     return errorlevel
   end