Search distribution and grep distribution inputs should not rely on placeholder for name

[sandbergja]

Describe the bug

When you are looking at a module page, there are two search fields on the left: "Search distribution" and "grep distribution". The fields are indicated only with a placeholder, which has a variety of accessibility issues.

To Reproduce

1. Go to https://metacpan.org/pod/Test2
2. In the "Search distribution" field, type "plan"
3. In the "grep distribution" field, type "plan"
4. Take a few minutes to get yourself some coffee or tea.
5. Look at your screen, and see if you can tell which field is "Search", and which one is "grep" without deleting what you wrote, or without relying on a lot of experience with these fields.

Expected behavior

I would expect to have:

• visible labels associated with these fields
• the labels are visible at all times, regardless of whether I have typed anything
• the labels have a color contrast ratio of 4.5:1 or higher to their background color

Desktop (please complete the following information):

• OS: Windows
• Browser: Firefox
• Version 127