Added script to generate altsrc flag definitions, restricted list of …

…types to the ones that provide implementation of FlagInputSourceExtension. The original program to generate altsrc flags is nowhere to be found, so replaced with fg.py. The types of altsrc flags are now limited to the subset that provide FlagInputSourceExtension implementation and have matching methods in InputSourceContext interface. Removed types where never updated from the input source.

altsrc/fg.py

@@ -0,0 +1,45 @@
+# Only types that provide implementation of FlagInputSourceExtension can be listed here
+# please keep list sorted alphabetically
+flag_types = [
+    "Bool",
+    "Duration",
+    "Float64",
+    "Generic",
+    "Int",
+    "IntSlice",
+    "Path",
+    "String",
+    "StringSlice",
+]
+
+print('''// Code generated by fg.py; DO NOT EDIT.
+
+package altsrc
+
+import (
+	"flag"
+
+	"github.com/urfave/cli/v2"
+)''')
+
+for t in flag_types:
+    print(f'''
+// {t}Flag is the flag type that wraps cli.{t}Flag to allow
+// for other values to be specified
+type {t}Flag struct {{
+	*cli.{t}Flag
+	set *flag.FlagSet
+}}
+var _ FlagInputSourceExtension = (*{t}Flag)(nil)
+
+// New{t}Flag creates a new {t}Flag
+func New{t}Flag(fl *cli.{t}Flag) *{t}Flag {{
+	return &{t}Flag{{{t}Flag: fl, set: nil}}
+}}
+
+// Apply saves the flagSet for later usage calls, then calls
+// the wrapped {t}Flag.Apply
+func (f *{t}Flag) Apply(set *flag.FlagSet) error {{
+	f.set = set
+	return f.{t}Flag.Apply(set)
+}}''')