apkbuild parser: support depends="$depends ..." (!1795)

Properly handle the following two cases in APKBUILDs:
* depends="$depends ..."
* depends="${depends} ..."

First I've attempted to refactor the parsing code to do this in a more
generic way. But I've realized that it would make more sense to retire
the python based APKBUILD parsing approach altogether and finally use a
shell script parser. Let's discuss this in #1801.

pmb/parse/_apkbuild.py

@@ -170,6 +170,13 @@ def apkbuild(args, path, check_pkgver=True, check_pkgname=True):
                 i += 1
                 line_value = lines[i][:-1]
 
+            # Support depends="$depends hello-world" (#1800)
+            if attribute == "depends" and ("${depends}" in value or
+                                           "$depends" in value):
+                previous = " ".join(ret["depends"]) if "depends" in ret else ""
+                value = value.replace("${depends}", previous)
+                value = value.replace("$depends", previous)
+
             # Split up arrays, delete empty strings inside the list
             if options["array"]:
                 if value:

test/test_parse_apkbuild.py

@@ -72,3 +72,9 @@ def test_kernels(args):
     ret = {"downstream": "Downstream description",
            "mainline": "Mainline description"}
     assert func(args, device) == ret
+
+
+def test_depends_in_depends(args):
+    path = pmb_src + "/test/testdata/apkbuild/APKBUILD.depends-in-depends"
+    apkbuild = pmb.parse.apkbuild(args, path, check_pkgname=False)
+    assert apkbuild["depends"] == ["first", "second", "third"]

test/testdata/apkbuild/APKBUILD.depends-in-depends

@@ -0,0 +1,8 @@
+pkgname="depends-in-depends"
+pkgver="1.0.0"
+pkgrel=0
+arch="armhf"
+depends="first"
+depends="$depends second"
+depends="${depends} third"
+pkgdesc="depends-in-depends test"