internal/rosa/python: append to source path

This gets around messy projects with multiple packages.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/rosa/python.go

@@ -83,6 +83,8 @@ func init() {
 
 // PipHelper is the [Python] pip packaging helper.
 type PipHelper struct {
+	// Path elements joined with source.
+	Append []string
 	// Whether to omit --no-build-isolation.
 	BuildIsolation bool
 	// Whether to enter source after install.
@@ -133,7 +135,7 @@ pip3 install \
 	--no-index \
 	--prefix=/system \
 	--root=/work \` + extra + `
-	'/usr/src/` + name + `'
+	'` + AbsUsrSrc.Append(name).Append(attr.Append...).String() + `'
 ` + script
 }